Better filename resolution.
Make signs optional (on by default).
Whitespace.
Check for grep

Fixes #72
Update only visible buffers on `FocusGained` and `TabEnter`.
Make grep optional instead of required.

When grep is available we use it for efficiency.  When it's unavailable,
we filter in Vimscript.
Allow plugin to bypass any alias for grep.

This is an opt-in configuration.
Merge branch 'master' of https://github.com/airblade/vim-gitgutter

Conflicts:
	plugin/gitgutter.vim