Can't import SublimeLinter at the top level, that causes multiple imports of sublimelinter.py
Added 'Lint This View' palette command with keyboard equivalent
Added support for lint modes

This required doing a bunch of work to make sure things are cleared out properly when not in background mode.
Fixed the choose commands to work correctly when nothing is selected and to do nothing when the current item is chosen
Make sure view-related data is deleted when a view is closed
Factored much of choose setting commands into a superclass