add 'list-prompts' and 'change-prompt' commands.

the list-prompts command shows a list of available prompts as well
as the active prompt in the list. prompts are drawn with a name as
well as a short description.

change-prompt can be used to change the prompt by the name found in
the list-prompts command.

screenshots:
![screen shot 2014-03-19 at 14 04 15](https://f.cloud.github.com/assets/77573/2460394/b5d5cf6a-af69-11e3-9c04-67755ec10fe3.png)
![screen shot 2014-03-19 at 14 24 24](https://f.cloud.github.com/assets/77573/2460399/d202e592-af69-11e3-81ee-2c2989c564cf.png)