Add support of IF [NOT] EXISTS for ADD/DROP COLUMN in Postgresql

Add support of IF [NOT] EXISTS for ADD/DROP COLUMN in Postgresql

Added :paramref:`.Operations.add_column.if_not_exists` and
:paramref:`.Operations.drop_column.if_exists` to render ``IF [NOT] EXISTS``
for ``ADD COLUMN`` and ``DROP COLUMN`` operations, a feature available on
some database backends such as PostgreSQL, MariaDB, as well as third party
backends.  The parameters also support autogenerate rendering allowing them
to be turned on via a custom :class:`.Rewriter`. Pull request courtesy of
Louis-Amaury Chaib (@lachaib).

Fixes: #1626
Closes: #1627
Pull-request: https://github.com/sqlalchemy/alembic/pull/1627
Pull-request-sha: c503b049f453a12e7fdb87464606cddd58ad306c

Change-Id: I4d55f65f072b9f03698b2f45f066872b5c3e8c58