Additional toxgen options

[sentrivana]

Sometimes you just want to make a change to some part of tox.jinja that is not toxgen-generated. In order to propagate this to tox.ini, you now can either do it manually (copy-paste) or run generate-test-files, but the latter will also attempt to update all toxgen-managed test definitions.

It'd also be good to have a special mode for when all you want is to add a new integration managed by toxgen, without regenerating all the rest.