Support simple blocks in functions

This adds support for simple blocks within functions, for example:

    calc((100% - 1px) / 2)

I have a large project I would like use use `csslint` on, but it has trouble parsing the CSS do to complex `calc` blocks. This is my attempt at adding CSS simple block support to this library. I restricted CSS simple block to just functions for now to reduce the impact.

See https://github.com/stubbornella/csslint/issues/267