- Added a new option

- Added a new option
:paramref:`.EnvironmentContext.configure.literal_binds`, which
will pass the ``literal_binds`` flag into the compilation of SQL
constructs when using "offline" mode.  This has the effect that
SQL objects like inserts, updates, deletes as well as textual
statements sent using ``text()`` will be compiled such that the dialect
will attempt to render literal values "inline" automatically.
Only a subset of types is typically supported; the
:meth:`.Operations.inline_literal` construct remains as the construct
used to force a specific literal representation of a value.
The :paramref:`.EnvironmentContext.configure.literal_binds` flag
is added to the "offline" section of the ``env.py`` files generated
in new environments.
fixes #255
- enhance the op_fixture as well as MigrationContext._stdout_connection()
 so that it uses the real DefaultImpl
and MigrationContext fully in tests.