Add a plugin to make sure there are no large files in a PR

[astrofrog]

We can check that each commit doesn't add more than a certain threshold in size, which would catch the case where a big file is removed in a subsequent commit but still lives in the history.

[voyager1001]

Hey @astrofrog , I'm interested in Open Source and GSoC. I'd like to work on this issue. Can I go forward with this ?

[Cadair]

I am not sure Baldrick is the best place for this there is already a pre-commit hook which does it.