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.
Merge pull request #1175 from johnny5-/list-prompts

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