Add optional live mode update delay for blocking mode

Prevents slowing down the editor by updating only at certain intervals instead of after every change. Easily tested by simply holding down a key in the editor with different delay values.

May work well in non blocking mode as well to save some resources but that
is not added since I don't have access to Sublime Text 3 to test it with.
Add delay setting and clarify non_blocking limitation.
Fix some errors in the comments
Catch AttributeError exception on self.view.window().run_command('git_gutter') when a delayed update is run after switching to another file
Update (near) instantly at a limited rate instead of always delaying. Feels snappier.

Added live_instant_interval and live_continuous_delay setting. live_delay
is now a bool and disabled by default.