Mapping Override Feature with Example Theme

- implemented ability to define mapping overrides in theme specification files
- added new example theme that uses the new function "Sublime Text Monokai Extended"
I'm a bit worried about these colours being theme keys - they could clash with standard theme keys in the future. Have you looked into how much effort it would be to have keys that will only apply locally?

This way, prefixing them with something from the theme name (like you did) is going to be a must.