Add the ability to disable evaluation on view focus change

In my work environment, using GitGutter on ST2 (where I already have the ```live_mode``` disabled), GitGutter hanged a lot due to the evaluation done when another view gets the focus, and it was blocking the whole application.

This adds a configuration option ```focus_change_mode``` to allow to disable the evaluation done on focus change.

I tried to enable evaluation at file loading instead, but it doesn't seem to work 100% of the times I open files. I'm not really familiar with Sublime's API but it seemed to me, seeing the docs, that it was the right way of doing it