- Added new argument

- Added new argument
:paramref:`.EnvironmentContext.configure.user_module_prefix`.
This prefix is applied when autogenerate renders a user-defined type,
which here is defined as any type that is from a module outside of the
``sqlalchemy.`` hierarchy.   This prefix defaults to ``None``, in
which case the :paramref:`.EnvironmentContext.configure.sqlalchemy_module_prefix`
is used, thus preserving the current behavior. #171
- added new documentation sections regarding rendering of types
- improved CSS so that deprecations/version changes are highlighted
- cleanup of some configure paramter docs