Push / Pull hooks.

[saturnodigital]

Hi @badele!

I'm making this PR not be accepted at all, but to explore the direction that we're going to.
I mean, I tried to implement the hook system very quick and dirty just to give us a way to check if it's what we're looking for at all.

I don't really know if I get you idea correctly (I think I was...), but I found the hook system too broad and general to be really useful (at least for what I was thinking about it). I mean the way that I implemented it, it gives too little context about the repository for me work with.

For example, I just have the repo path (name) and with that I'll need roll all my scripting to iterate in the repo's branches, etc. gitcheck is already doing that in it's default operation.
I would prefer that gitcheck call my hook for each branch that needs a push or a pull instead, this way I'll need roll out the scripting to check if it should actually pull, merge, push, etc.

I hope that I made myself clear, and sorry for the delay to implement this, I had some very busy time the last two weeks!

PS: This is not a PR to be accepted so it's missing a lot of stuff that I'll be included, don't pay attention to that, but to the general idea!

Thanks!