Feature: Highlight non-printable characters

Adds a new `-A`/`--show-all` option (in analogy to GNU Linux `cat`s option) that
highlights non-printable characters like space, tab or newline.

This works in two steps:
- **Preprocessing**: replace space by `•`, replace tab by `├──┤`, replace
newline by `␤`, etc.
- **Highlighting**: Use a newly written Sublime syntax to highlight
these special symbols.

Note: This feature is not technically a drop-in replacement for GNU `cat`s
`--show-all` but it has the same purpose.

src/clap_app.rs

@@ -158,6 +158,18 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                      '--style=numbers'",
                 ),
         )
+        .arg(
+            Arg::with_name("show-all")
+                .long("show-all")
+                .alias("show-nonprintable")
+                .short("A")
+                .conflicts_with("language")
+                .help("Show non-printable characters (space, tab, newline, ..).")
+                .long_help(
+                    "Show non-printable characters like space, tab or newline. \
+                     Use '--tabs' to control the width of the tab-placeholders.",
+                ),
+        )
         .arg(
             Arg::with_name("line-range")
                 .long("line-range")