Added lighten/darken support

In the spirit of full reverse-compatibility with stylus, I added `lighten/darken` support.

You can now do `shade(color, amount)` or `lighten(color, amount)` or `darken(color, amount)` which is equivalent to `shade(color, -amount)`. Lastly, I added optional % support for the amount, since Stylus supports it. So you can do `lighten(color, 34%)` or `lighten(color, 34)`.