Extract css attribute scrubbing to its own method.

Hi Mike.

We're still working on integrating Loofah with Rails.
We have a custom scrubber, that lets you add both tags and attributes to allow when sanitizing. If people have added the style attribute, we don't want `scrub_attributes` to scrub that.
So I've added a `scrub_css_attribute` method.

Here's where we're using it: https://github.com/rafaelfranca/rails-html-sanitizer/blob/master/lib/rails/html/scrubbers.rb#L105