Pass payload primitive values as gh_<key> env vars

[kapouer]

So one can exec a string like (on a create event, for example): git fetch && git checkout ${gh_ref}

[kapouer]

Refactored and extended, so one can do stuff like

{
  "event": "create",
  "match": "ref_type == 'tag'",
  "exec": "cd ~/src/${gh_repository_name}/site/; git fetch; git checkout ${gh_ref}; make install restart"
}

[rvagg]

@kapouer I merged this and published a semer-major with it yesterday, reformatted a bit,