Render `if_not_exists` option for CreateTableOp, CreateIndexOp, DropTableOp and...

Render `if_not_exists`  option for CreateTableOp, CreateIndexOp, DropTableOp and DropIndexOp

Render ``if_exists`` and ``if_not_exists`` parameters in
:class:`.CreateTableOp`, :class:`.CreateIndexOp`, :class:`.DropTableOp` and
:class:`.DropIndexOp` in an autogenerate context.  While Alembic does not
set these parameters during an autogenerate run, they can be enabled using
a custom :class:`.Rewriter` in the ``env.py`` file, where they will now be
part of the rendered Python code in revision files.  Pull request courtesy
of Louis-Amaury Chaib (@lachaib).

Closes: #1446
Pull-request: https://github.com/sqlalchemy/alembic/pull/1446
Pull-request-sha: 90c9735767af1cf3ba7e40e71dfa0fb30efc1ee8

Change-Id: I6b0a5ffaf7e2d1a0a1e1f1e80ed0ee168ae2bd09