Notify.pm: support running custom notify hook

Notify.pm: support running custom notify hook

Allow a custom notify hook to be configured (by default with the
new $Girocco::Config::default_notifyhook config) and/or per-repository
with the new girocco.notifyHook config.

If configured, the hook will be run immediately before all other
notify processing takes place for that particular batch of ref
changes.

All errors and non-0 exit status from running the notify hook are
completely ignored and do not prevent the normal notifications that
would come after it from being performed.

Hook semantics are basically the same as for Git's post-receive
hook with the addition of four command line arguments and extra
"context" lines.

The hook value gets interpreted and "run" similarly to how Git
"runs" the value of the GIT_EDITOR and similar environment variables.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>