Implement syntax mapping

This adds a `-m`/`--map-syntax` option that allows users to (re)map
certain file extensions or file names to an existing syntax.

For example:
```
bat --map-syntax .config:json
```

The option can be use multiple times. Note that you can easily make
these mappings permanent by using `bat`s new configuration file.

closes #169

src/printer.rs

@@ -142,7 +142,7 @@ impl<'a> InteractivePrinter<'a> {
             };
 
             // Determine the type of syntax for highlighting
-            let syntax = assets.get_syntax(config.language, file, reader);
+            let syntax = assets.get_syntax(config.language, file, reader, &config.syntax_mapping);
             Some(HighlightLines::new(syntax, theme))
         };