Integrate gitgutter with xolox's vim-shell to avoid the flash on windows taskbar

The feature is disabled by default. When it is disabled, there are only few changes between this one and the original, thus I'm pretty sure it should work without issue.

This feature will be enabled **only if** `vim-shell`, `vim-misc` by @xolox are installed, and the user want to enable it by:
```vim
let g:gitgutter_avoid_cmd = 1
```
It works fine so far. Even if there are issues, at lease we can turn the feature off before I can fix it. :)