Feature: Update known/unsupported model list based on API

[jschuur]

• [x] on first use, update the list of models via the apI
• [x] store this lis in config
• [x] validate against it when switching models via .model command
• [x] also periodically (once a week?) update this list
• [x] .models update command forces an update
• [x] when you try and switch to an unknown model, remind about .models update` (but still allow it?)

[jschuur]

Completed for 0.5.0