Come up with a standardized interface for user interaction

[pythonesque]

Thinking about things like saving a file, displaying errors, etc., we should have a standard interface for communicating with the user. Things to think about:

• loading / saving a file
• displaying an error (short or long form)
• yes/no prompt
• selecting a buffer

@gchp mentioned that vigo has the concept of an Overlay. We should look into that.

[gchp]

Created a PR for this. It needs some more work still, and can probably be generalised a bit more to make it more re-usable, but its functional. Check it out and see what you think