Support explicit column ordering in batch mode

Support explicit column ordering in batch mode

Added new parameters :paramref:`.BatchOperations.add_column.insert_before`,
:paramref:`.BatchOperations.add_column.insert_after` which provide for
establishing the specific position in which a new column should be placed.
Also added :paramref:`.Operations.batch_alter_table.partial_reordering`
which allows the complete set of columns to be reordered when the new table
is created.   Both operations apply only to when batch mode is recreating
the whole table using ``recreate="always"``.  Thanks to Marcin Szymanski
for assistance with the implementation.

Co-Authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #640
Closes: #646
Pull-request: https://github.com/sqlalchemy/alembic/pull/646
Pull-request-sha: 29392b796dd995fabdabe50e8725385dbe1b4883

Change-Id: Iec9942baa08da4a7fc49b69093e84785fea3cec2