Add draft functionality to `content_api.tags`.

This param is available for all content_api endpoints but is currently not used except on #tags.

Part of https://www.pivotaltracker.com/story/show/73971142
This method looks like it will have weird behaviour if `:type`, `:live` and `:draft` keys aren't provided. Rather than failing late in possibly unclear ways can we pre-emptively `fetch` them, thus blowing up with a `KeyError` which should be pretty clear.