Syntax-highlight output using Pygments

Building on the earlier pull request which allows suffixes, this one introduces a dependency on Pygments, and causes cheatsheets with a known language suffix (e.g. `audio.py`) to be syntax highlighted in the console using the appropriate lexer.

The default lexer, when there is no suffix, highlights shell syntax. If a suffix exists, but no known lexer is found, it will default to text (i.e. no highlighting).