Add support for latest releases from Gitea/Forgejo

[pieterhijma]

This pull-request adds support for automatic inclusion of the latest release of projects hosted on Gitea/Forgejo instances such as Codeberg.org, see issue #23. It allows the following syntax in source.txt:

gitea-latest-release <git repository on a gitea instance>

An example would be:

latest-gitea-release https://codeberg.org/osh-autodoc/cl-scribble-subset-parser.git

The addition is based on the code for Gitlab and it was tested on Quicklisp Projects.

Thanks in advance for considering this pull request.

[moedn]

@quicklisp would be lovely to have this merged :)