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"
To me, `parseMapping` sounds like something that's called multiple times from `parseMappings` - not like something that actually calls `parseMappings`. How about calling `parseMappings` something like `parseStandardMappings` instead and renaming `parseMapping` to `parseMappings`?