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/main.rs

@@ -35,6 +35,7 @@ mod output;
 mod preprocessor;
 mod printer;
 mod style;
+mod syntax_mapping;
 mod terminal;
 mod util;