Today I implemented [[external backends|design/external_backend_protocol]]
for keys. So unusual new hashes can be used by writing a small program.

Probably lots of other uses for this too; I don't know if I'll like
them all. It has the potential to warp git-annex in some directions I don't
want to deal with. Still, it's good to have this feature.

I was able to reuse a lot of the external special remote code for this,
and only had to write around 400 lines of new code. Dunno how that all
happened in 8 hours, but it did!
