#71 prompt: doesn't have _teardown() method

[Christian Höltje]

The prompt system has a prompt_<name>_setup method used to setup the prompt.

It really should have a teardown method, too.

Some prompts setup things like precmd and prexec hooks (using zsh-add-hook) and should be allowed to remove those hooks.

Alternatively, prompt could just auto-delete all the methods that begin with 'prompt_<name>*'.

Or both, actually, would be good.