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/printer.rs

@@ -22,7 +22,7 @@ use diff::get_git_diff;
 use diff::LineChanges;
 use errors::*;
 use inputfile::{InputFile, InputFileReader};
-use preprocessor::expand;
+use preprocessor::expand_tabs;
 use style::OutputWrap;
 use terminal::{as_terminal_escaped, to_ansi_color};
 
@@ -177,7 +177,7 @@ impl<'a> InteractivePrinter<'a> {
 
     fn preprocess(&self, text: &str, cursor: &mut usize) -> String {
         if self.config.tab_width > 0 {
-            expand(text, self.config.tab_width, cursor)
+            expand_tabs(text, self.config.tab_width, cursor)
         } else {
             text.to_string()
         }