Add GitLab formatter

Adds the HTML formatter used by [GitLab](gitlabhq/gitlabhq). (also see gitlabhq/pygments.rb#1)
* produces more lightweight HTML code
* allows for linking/highlighting of lines after rendering is done (see gitlabhq/gitlabhq#1937)
* the formatter itself is a lot simpler
* makes packaging GitLab easier for distos :wink: (see https://github.com/gitlabhq/pygments.rb/pull/1#commitcomment-3399722)