add check command for upgrade diffs

add check command for upgrade diffs

Added new Alembic command ``alembic check``. This performs the widely
requested feature of running an "autogenerate" comparison between the
current database and the :class:`.MetaData` that's currently set up for
autogenerate, returning an error code if the two do not match, based on
current autogenerate settings. Pull request courtesy Nathan Louie.

As this is a new feature we will call this 1.9.0

Fixes: #724
Closes: #1101
Pull-request: https://github.com/sqlalchemy/alembic/pull/1101
Pull-request-sha: 807ed545df70e7a10b913e2951a1b636f138a4ff

Change-Id: I03b146eaf762be464a0ff0858ff5730cc9366c84