Allow syncing from git repo

[ewinslow]

See my previous PR: https://github.com/Elgg/community_plugins/pull/8

Git tags in a valid php version format or vX.Y.Z format would automatically become releases.

[hypeJunction]

This would only work with plugins that include all their dependencies. What happens to plugins that rely on composer, npm and bower?

[ewinslow]

npm/bower deps would have to be checked in. Expectation is that people could/would use composer to install if you are using composer to track dependencies.