Git pull support

[ramonGonzEdu]

Problem description
Auto-updating mods downloaded from github instead of steam.

Describe the solution
There should be a button on locally installed mods that have a .git subfolder. Pressing this button would run git pull in the mod directory, downloading the newest version from github or whatever remote is set. This button shouldn't appear if git isn't installed.

Alternatives
Manually going into the folders and running git pull works pretty well, and a small script can somewhat automate it.