Add markers to track error lines

Keeps track of marks to prevent re-creating the same mark multiple times and so it can properly destroy marks (and their decorations) when they are no longer valid.

Should fix https://github.com/sindresorhus/atom-jshint/issues/32

It's entirely possible that I missed some use-case here, so other eyes are appreciated.
I'd prefer to somehow have a local markers object for each editor instead of a global object.