Allow overriding of script.py.mako path in generate_revision

[gricey432]

We have a use case where our script.py.mako is stored in a central location to be shared by multiple sets of alembic revisions. We currently just copy it into each project's scripts directory but that's not ideal.

It looks like this could be allowed by adding an optional kw arg to generate_revision which is then passed into _generate_template, using the existing value as a default.

[zzzeek]

it has to be passed in all the way from the environment context, I can accept a PR that includes passing tests.