Refactor `CommandLine` command registration
Added new :meth:`.CommandLine.register_command` method to
:class:`.CommandLine`, intended to facilitate adding custom commands to
Alembic's command line tool with minimal code required; previously this
logic was embedded internally and was not publicly accessible. A new
recipe demonstrating this use is added. Pull request courtesy Mikhail
Bulash.
Fixes: #1610
Closes: #1611
Pull-request: https://github.com/sqlalchemy/alembic/pull/1611
Pull-request-sha:
d2ffce008317508449de9bd0bce3ff9076c5d90f
Change-Id: Icab9978357915e81bb52a9f0717f21d5ee7f8341