refactor: removes unused imports on nightly via cfg directive

If one is using a nightly Rust compiler they should compile clap
with the `nightly` feature.

```toml
[dependencies]
clap = { version = "2.27", features = ["nightly"] }
```

This isn't required for compilation to succeed, only to take
advantage of any nightly features used by clap.

If one is compiling with `#[deny(warnings)]` this commit will cause
compilation to fail if one *does not* compile with the `nightly`
since `std::ascii::AsciiExt` is no longer required in in multiple
files as of the latest Rust nightly (Nov 6th, 2017).

Closes #1095

src/app/settings.rs

@@ -1,4 +1,5 @@
 // Std
+#[cfg(not(feature = "nightly"))]
 use std::ascii::AsciiExt;
 use std::str::FromStr;
 use std::ops::BitOr;

src/args/settings.rs

@@ -1,4 +1,5 @@
 // Std
+#[cfg(not(feature = "nightly"))]
 use std::ascii::AsciiExt;
 use std::str::FromStr;
 

src/completions/shell.rs

@@ -1,3 +1,4 @@
+#[cfg(not(feature = "nightly"))]
 use std::ascii::AsciiExt;
 use std::str::FromStr;
 use std::fmt;

src/completions/zsh.rs

@@ -1,6 +1,7 @@
 
 // Std
 use std::io::Write;
+#[cfg(not(feature = "nightly"))]
 use std::ascii::AsciiExt;
 
 // Internal

src/macros.rs

@@ -308,6 +308,7 @@ macro_rules! arg_enum {
             type Err = String;
 
             fn from_str(s: &str) -> ::std::result::Result<Self,Self::Err> {
+                #[cfg(not(feature = "nightly"))]
                 use ::std::ascii::AsciiExt;
                 match s {
                     $(stringify!($v) |

tests/macros.rs

@@ -147,3 +147,15 @@ fn quoted_arg_name() {
         .expect("Expected to successfully match the given args.");
     assert!(matches.is_present("option2"));
 }
+
+#[test]
+fn arg_enum() {
+    arg_enum!{
+        #[derive(Debug, PartialEq, Copy, Clone)]
+        pub enum Greek {
+            Alpha,
+            Bravo
+        }
+    }
+    assert_eq!("Alpha".parse::<Greek>(), Ok(Greek::Alpha));
+}