Optionally use mkdirp to ensure output destination directory exists.

`mkdirp` is a very widely used module, trivial in size, and I probably should have used it in the first place.

I would write tests, but there are currently none written for the `lessc` utility. I would feel better about doing so if there was a recognizable testing framework already in use (mocha, nodeunit, vows, etc). Rest assured, `mkdirp` performs as advertised.

This fixes issue #1099