Use a context handler for file history overlay commands

Instead of using a transient setting in Preferences.sublime-settings,
a custom context handler keeps track of the state of the file history
panel.
Use a context handler for file history overlay commands

Instead of using a transient setting in Preferences.sublime-settings,
a custom context handler keeps track of the state of the file history
panel.
Merge branch 'Feature/ContextHandler' of github.com:rahul-ramadas/sublimetext-filehistory into Feature/ContextHandler

Conflicts:
	file_history.py
No need to track history overlay per-window

There is an instance of a WindowCommand for each window. However, only
the currently focused window can have an open overlay, so for our
purposes there is no need to track per-window.
Merge pull request #17 from rahul-ramadas/Feature/ContextHandler

Merging the use of a custom context handler rather than hijacking the main preferences