Feature/subtree autotagging

Replaces https://github.com/elcodi/github-webhook/pull/1

* Splitting repo is done via a one pass process that uses `subtree split` instead of `filter-branch`
* Tagging is done in the same process when a `tag` is found in the `HEAD` of the `elcodi/elcodi` master repo
* Other features listed in https://github.com/elcodi/github-webhook/pull/1 (`cs-fixer`, `.gitignore`)