Live selection of LLM server

[nbloomf]

Right now the plugin supports multiple backends, but only one can be used at a time (selected by command line flag at invocation time).

There is no reason this can't be configurable so that the developer can switch between, say, a cheap local model and an expensive remote model on the fly.

[nbloomf]

WIP: #29

[nbloomf]

(forgot to close this)