Add an ANSI formatter for output on the console

To use the ANSI formatter, the 'format' option has to be set to 'ansi'

See it in action at https://bit.ly/luminous-ansi-demo

Features:
- Based on the LaTeX formatter (well, that’s not a feature, but I think it’s worth mentioning)
- Support for XTerm 256 colors both as text color and as background color
- Support for bold, italic, underline, and strikethrough

Known issues which will be fixed when the conversion to PSR (#31) is finished:
- line numbers are not yet supported
- Nested tokens (like escape sequences inside strings) are not yet supported.