Add Less language

Support for [Less](http://lesscss.org/) language.

List of known issues and limitations (just for reference):
- Comments with `{` in a rule statement make this statement to be highlighted as a selector 
- CSS [escaping](http://www.w3.org/TR/CSS2/syndata.html#escaped-characters) (i.e. `\`) is not quite supported (so `"\0025"` is colored properly but `"foo\"bar"` is not) - too rarely used feature to bother
- `all` is colored as keyword even when used as tag (too minor issue to encumber things with)
- `%` function name is not highlighted (too minor to bother)
- `@page`'s `ident:pseudo` selectors are not properly highlighted (too subtle issue to burden the parser further)
- Subsequent `@page` "Box"-rules (e.g. `@top-left`, `@bottom-right-corner` etc.) are highlighted as variable definitions (too scarce feature)
- Definition of variable with at-rule name (e.g. `@media: 1px;`) is colored as an at-rule (too rare to care)