Add custom kicker

Allows editors to add a custom kicker to an item:

![screen shot 2014-10-01 at 16 06 27](https://cloud.githubusercontent.com/assets/1001642/4477284/cd64b288-497c-11e4-9408-f1784ed705e9.png)

![screen shot 2014-10-01 at 16 06 36](https://cloud.githubusercontent.com/assets/1001642/4477285/d02f24bc-497c-11e4-89a7-5d0af1cb9559.png)

![screen shot 2014-10-01 at 16 06 42](https://cloud.githubusercontent.com/assets/1001642/4477287/d2bd722e-497c-11e4-97d4-bcf8bf7ed784.png)

CC @stephanfowler @kungpochicken