Add support for mono-repositories

[satazor]

The current implementation does not work with mono-repos:

1. It assumes .git is in the cwd
2. It doesn't allow filtering releases. In a mono-repo, there are different releases and tags per package (usually prefixed by the package name, like my-package@v1.0.0
3. It doesn't allow filtering pull-requests by changes files, so that only pull-requests that affected a package are considered.

I've fixed 1 and implemented options for 2 and 3.

Example usage:

github-changelog-generator --future-release my-service@v0.1.0 --release-tag-prefix my-service@v