Support git archives

[Changaco]

git can inject data into archives, including tags. Here's how it could work:

1. put use_scm_version={'git_metadata': '$Format:%D$'} in setup.py
2. put setup.py export-subst in .gitattributes
3. setuptools_scm extracts the version number from git_metadata with a regexp like \btag: ([0-9][^,]*)\b

[RonnyPfannschmidt]

The metadata would be incomplete and broken in many situations

Also in that case a file named .git_archival.txt should be used and registered as scm backend
I'm happy to include a Variant that demonstrably works correct with random archives from github

But I will not again spend time researching and implementing it for git

[RonnyPfannschmidt]

will be solved by #67 which should document the plugin that was kindly implemented and released by the original reporter