implement public-inbox-purge tool

implement public-inbox-purge tool

Expose the ->purge functionality of V2Writable for rewriting
git history to permanently purge messages from history.  This
may be necessary for legal reasons.

Usage:

	# requires ~/.public-inbox/config
	public-inbox-purge --all </path/to/message-to-purge

	# good for testing with unconfigured inboxes:
	public-inbox-purge $INBOX_DIR </path/to/message-to-purge