Don't show error on empty REPL command.

Instead, just give a new prompt.
Docstring
Major simplification of marker protocol in REPL buffer

This makes it easier to make the prover interact well with the REPL.
Make prover window more reliable
Update metavariable list after proving something
Add missing cleanup to idris-quit.
Detect loading while proof active

Then prompt to abandon it.
Merge pull request #180 from david-christiansen/prover

Prover usefulness