Add dialect_options to environment; set paramstyle=named for offline

Add dialect_options to environment; set paramstyle=named for offline

Fixed bug where the double-percent logic applied to some dialects such as
psycopg2 would be rendered in ``--sql`` mode, by allowing dialect options
to be passed through to the dialect used to generate SQL and then providing
``paramstyle="named"`` so that percent signs need not be doubled.   For
users having this issue, existing env.py scripts need to add
``dialect_opts={"paramstyle": "named"}`` to their offline
context.configure().  See the ``alembic/templates/generic/env.py`` template
for an example.

Change-Id: Ia6a495704b029eaff43fb3b6df602ca667002b7a
Fixes: #562