Feature/branch scripts

First of all, great tool. We've been using it and are pretty happy with it.

This PR makes it possible to call different build and publish scripts based on which branch is firing the webhook. One of the use cases would be to build and publish a ```develop``` branch to a staging environment with simple authentication, while ```master``` would trigger a build and publish to a production environment.

The other commit makes it possible to use Jekyll Hook with branch names with a slash in them, like ```feature/pretty_map```.
That's part of branch name fix. Using a named parameter was a problem with branches like ```feature/whatever```. The slash in the branch name was causing the branch parameter not to work. By setting the path to ```/hooks/jekyll/*``` we need to use a numeric index to get the value.