Add option to diff files; improve argument parsing

Closes #10

Cargo.toml

@@ -5,5 +5,7 @@ authors = ["Nathan Kessler <[email protected]>"]
 edition = "2018"
 
 [dependencies]
+clap = "2.33"
+difference = "2.0"
 walkdir = "2"
 regex = "1"

README.md

@@ -10,6 +10,7 @@ Storing dotfiles in git repositories allows them to be shared across multiple co
 * Toggle chunks of files per feature flags.
 * Copy binary files without templating.
 * Preserve file permissions.
+* Diff mode to compare expected output against existing files
 
 ## Planned Features
 Feature requests are welcome!
@@ -28,6 +29,12 @@ dot-templater CONFIG SRC_DIR DEST_DIR
 
 Copies files from `SRC_DIR` to `DEST_DIR` according to rules in `CONFIG`.
 
+```
+dot-templater --diff CONFIG SRC_DIR DEST_DIR
+```
+
+Compares files from `SRC_DIR` modified according to rules in `CONFIG` against the contents of `DEST_DIR`.
+
 ### Config Format
 Any line beginning with `#` is ignored. Config file can contain key/value substitutions and feature flags.