Support custom global git hooks

This enables administrators to create their own git hooks in `/home/git/gitlab-shell/hooks` which will be sym-linked into all new repositories managed by gitlab-shell. Because GitLab relies on an `update` hook I have created a wrapper which first calls the GitLab update hook and then the local update hook (if it exists). If the GitLab update hook fails the local hook is never executed.

Addresses #71, #72, #10, #14.

- [ ] integration tests
- [ ] update changelog
- [ ] update version
- [ ] document in README

/cc @randx @netdata