Make annex.addunlocked be able to be configured in .gitattributes,
the same way as annex.largefiles can be.

This would be useful if certain filename extensions need to be unlocked to
be used, and others are desired to be kept locked.

The annex.addunlocked git config is a boolean, but this gitattributes one
would effectively build up a file match expression. So it might then follow
that the git config should also be a file match expression, with "true"
being the same as "anything" and "false" the same as "nothing" for
back-compat. --[[Joey]]

> This got accomplished by other means, [[done]] --[[Joey]]
