Add LESS support (.less)

Cheating slightly as it uses the CSS lexer, as pygments currently does
not have a dedicated less lexer. But I figure language recognition and
90% percent correct syntax highlighting is better than neither.

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If the lexer is deemed to be ill-fitting then even using the "Text Only" lexer would be worthwhile, just so the primary language of Github's most popular repo (i.e [twitter/bootstrap](https://github.com/twitter/bootstrap)) can be recognized as such.