Implement the unique-id() function

This PR implements the `unique-id` function introduced in Ruby sass 3.3 by returning a random unique CSS identifier. The implementation is pretty much an exact copy of the Ruby sass implementation https://github.com/sass/sass/commit/f3be0f40b7c5072635b2d6dab772f2c29e8d629a.

This implementation is slightly different to the Ruby implement which

> Returns a unique CSS identifier within the scope of a single CSS file.

I'm not sure the scoping serves any particular benefit but maybe @chriseppstein knows better?

Fixes https://github.com/sass/libsass/issues/636. Not sure how best to spec this to be honest.
Looks like a Ruby trick for turning numbers into alpha numeric strings http://www.chris3000.com/ruby/ruby-how-to-turn-integers-into-base36-for-urls/

I'll drop this to 16 since we're passing through hex anyway.