Customisable kickers

@stephanfowler & I: Allows editors to choose a custom kicker based on the tag or section.

Next up is the free HTML kicker field.

![screen shot 2014-09-29 at 12 20 53](https://cloud.githubusercontent.com/assets/1001642/4440857/ba5b08da-47ca-11e4-961d-b16edd2a4dbf.png)

CC @gklopper @sndrs @janua 
NB although this is added in this commit, we don't have the option to actually create a free kicker in the tool yet. That'll be a separate pull request.