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: #1611
Pull-request-sha: d2ffce0

Change-Id: Icab9978357915e81bb52a9f0717f21d5ee7f8341

Author: mikeroll