Multiple backends

Now it's possible to switch the backend between the default chatgpt one, a local textgen, or a simple lorem generator. (It's intended, but not tested, that additional backends can be shipped independently)

Now all commands are sub-commands of chap: chap tui instead of chap-tui. (It's intended, but not tested, that additional commands can be shipped independently)

I think the problem of individual markdown elements sometimes getting a scrollbar is resolved, but it's hard to be confident.