Support if_exists and if_not_exists on create/drop table commands

Support if_exists and if_not_exists on create/drop table commands

Added support for :paramref:`.Operations.create_table.if_not_exists` and
:paramref:`.Operations.drop_table.if_exists`, adding similar functionality
to render IF [NOT] EXISTS for table operations in a similar way as with
indexes. Pull request courtesy Aaron Griffin.

Fixes: #1520
Closes: #1521
Pull-request: https://github.com/sqlalchemy/alembic/pull/1521
Pull-request-sha: 469be01c6b5f9f42dc26017040a6fc54c4caef54

Change-Id: I5dcf44d9e906cdb84c32c4bfb6a1c63cde6324fd