generate: Be smarter when no generate_merge setting is set

[mikix]

By default, i18n_tool extract creates django-partial.po and djangojs-partial.po.

But by default, i18n_tool generate does nothing with them. There should be a default config for generate_merge, like so:

generate_merge:
django.po:
- django-partial.po
djangojs.po:
- djangojs-partial.po

I don't know whether there should be a default setting for make.po and underscore.po. Those are already in reasonably-named files. But leaving files named 'partial' around seems bad. Especially when the rest of the tooling assumes you're merging into the final versions.