Better Labels output to HTML

Labels were extracted to its own parser module, as it's a different thing from the other references and the HTML output is now similar to how it is in GitLab.com. This would allow us to render labels as it is rendered in GitLab.com (sort-of).

This is the first step to https://gitlab.com/gitlab-org/gitlab_kramdown/issues/10, so we can render "something" that looks like a label, with a default color code.

In the future we should combine an additional datasource to fetch label data from (like a local yaml file, that can be feed from previous API requests).