Adds a generate command for a default .tools.toml file (chshersh#62)

Resolves chshersh#52.

The generate command also uses the --config flag to set the location of
the generated file. Would it be a good idea to have some output when
actually generating the config? Right now there is no actual response
unless there is some file system error.

```bash
$ cargo run -- --config test-tools.toml generate
   Compiling tool-sync v0.1.0 (/home/mitchell/rust/tool-sync)
    Finished dev [unoptimized + debuginfo] target(s) in 2.14s
     Running `target/debug/tool --config test-tools.toml generate``

$ cat test-tools.toml 
# This file was automatically generated by tool-sync
#
#store_directory = "$HOME/.tools.toml"
#
#    [bat]
#        owner =     "sharkdp"
#        repo =      "bat"
#        exe_name =  "bat"
#        tag =       "latest"
#    [exa]
#        owner =     "ogham"
#        repo =      "exa"
#        exe_name =  "exa"
#        tag =       "latest"
#    [fd]
#        owner =     "sharkdp"
#        repo =      "fd"
#        exe_name =  "fd"
#        tag =       "latest"
#    [ripgrep]
#        owner =     "BurntSushi"
#        repo =      "ripgrep"
#        exe_name =  "rg"
#        tag =       "latest"
```

README.md

@@ -100,22 +100,27 @@ store_directory = "~/.local/bin"
 [ripgrep]
 ```
 
-By default `tool-sync` reads configuration from `~/.tool.toml` but you can put
-the content in any place and specify the path via the `--config` flag.
+By default `tool-sync` reads configuration from `$HOME/.tool.toml` you can run `tool
+default-config` to print a default configuration example to std out. You can
+redirect this out put to a file like so `tool default-config >
+$HOME/.tools.toml`.
 
 You can also quickly copy the above configuration to the default path by running
 the following command (Unix-only):
 
 ```shell
 curl https://raw.githubusercontent.com/chshersh/tool-sync/main/example-tool-sync-config.toml > ~/.tool.toml
 ```
+A default config can be also be generated by running `tool --config=path/to/config generate`.
+This will generate an example file at `path/to/config`.
 
 The above example config lists some tools natively supported by `tool-sync` and
 therefore they don't require extra configuration.
 
 To specify a tool not supported by `tool-sync`, add a TOML table entry and list
 all the required fields like in the example below:
 
+
 ```toml
 [tokei]
 owner    = "XAMPPRocky"  # GitHub username

src/config.rs

@@ -1,3 +1,4 @@
 pub mod cli;
 pub mod schema;
+pub mod template;
 pub mod toml;

src/config/cli.rs

@@ -17,4 +17,7 @@ pub struct Cli {
 pub enum Command {
     /// Sync all tools specified in configuration file
     Sync,
+
+    /// Generate a default .tools.toml file and prints it to std out
+    DefaultConfig,
 }

src/config/template.rs

@@ -0,0 +1,38 @@
+/// This file only holds the template that is used to generate a default .tools.toml.
+
+pub const CONFIG_TEMPLATE: &str = r##"# # tool-sync default configuration file
+# https://github.com/chshersh/tool-sync
+# This file was automatically generated by tool-sync
+#####################################################
+#
+#
+# store_directory = "$HOME/.local/bin"
+#
+# tool-sync provides native support for some of the tools without the need to configure them
+# Uncomment the tools you want to have them
+#
+# [bat]
+# [difftastic]
+# [fd]
+# [ripgrep]
+#
+# To add configuration for other tools these are the config options:
+# [ripgrep]
+#        owner     = "BurntSushi"
+#        repo      = "ripgrep"
+#        exe_name  = "rg"
+#
+#        # Uncomment to download a specific version or tag.
+#        # Without this tag latest will be used
+#        # tag       = "13.0.0"
+#
+#
+# Asset name to download on linux OSes
+# asset_name.linux = "x86_64-unknown-linux-musl"
+#
+# uncomment if you want to install on macOS as well
+# asset_name.macos = "apple-darwin"
+#
+# uncomment if you want to install on Windows as well
+# asset_name.windows = "x86_64-pc-windows-msvc"
+"##;

src/lib.rs

@@ -7,26 +7,30 @@ use clap::Parser;
 use std::path::PathBuf;
 
 use crate::config::cli::{Cli, Command};
+use crate::config::template;
 use crate::config::toml;
 use crate::sync::sync;
 
 const DEFAULT_CONFIG_PATH: &str = ".tool.toml";
 
 pub fn run() {
     let cli = Cli::parse();
-    let config_path = resolve_config_path(cli.config);
+    let config_path = resolve_config_path(cli.config.clone());
 
-    match toml::parse_file(&config_path) {
-        Err(e) => {
-            err::abort_with(&format!(
-                "Error parsing configuration at path {}: {}",
-                config_path.display(),
-                e.display()
-            ));
-        }
-        Ok(tool) => match cli.command {
-            Command::Sync => sync(tool),
+    match cli.command {
+        Command::Sync => match toml::parse_file(&config_path) {
+            Err(e) => {
+                err::abort_with(&format!(
+                    "Error parsing configuration at path {}: {}",
+                    config_path.display(),
+                    e.display()
+                ));
+            }
+            Ok(tool) => {
+                sync(tool);
+            }
         },
+        Command::DefaultConfig => generate_config(),
     }
 }
 
@@ -46,3 +50,7 @@ fn resolve_config_path(config_path: Option<PathBuf>) -> PathBuf {
         },
     }
 }
+
+fn generate_config() {
+    println!("{}", template::CONFIG_TEMPLATE);
+}