pre-init config and hook

pre-init config and hook

Added annex.pre-init-command git config and pre-init-annex hook that is run
before git-annex repository initialization.

This can block initialization. Or it can preform pre-initialization
configuration or tweaking.

I left stdio connected while it's running, so it could also be used for
interactive prompting conceivably, although that would want to use /dev/tty
anyway probably in order to not pollute the stdout of a command when
automatic initialization is done.

Sponsored-by: Dartmouth College's OpenNeuro project