Add support for excluding south migrations

[nijel]

While using South and it's generated migrations, you'll get lot of automatically generated code with poor PEP-8 compliance, so it would be great to ignore them.

It could probably be done automatically once using Django (ignore */migrations/*.py) or providing some generic way to define files to be skipped from checking.

[carlio]

Hi @nijel - this should already be happening, and I can't reproduce the problem with the current master branch. However I have made several changes to the ignoring code recently so probably I have accidentally fixed the issue.

Is the repository you're using publicly available? If not, could you try using the current master branch and see if you still have this issue? Thanks!

[nijel]

The code is hosted at github - https://github.com/nijel/weblate

[carlio]

Great thanks, I'll take a look. Actually I think I just found the problem, it was not showing up for me due to a different bug!