Closes #30 

I decided not to reinvent the wheel and tried to use the existing mechanics with Less mixin parameters, and it seems to work.

Checking the selector with a regular expression looks fragile, but it doesn't break anything and covers the scenarios that people complain about. If you can think of a scenario that would break it, feel free to share.

Note: I added an additional test with the `indentInsideParens` option just to show the current behavior. It is expected that double indentation only occurs at the first level - we cannot do it differently without rewriting anyway, since the selector is not parsed here. It is simply treated as a string (along with the nested pseudo).

…or multiline pseudo-classes

Resolves #70