Customise reporting

[StefanBRas]

Hey!

I'm trying out diff_cover in Github Actions. Github Actions supports annotations where If I in a job run print out something like

::warning file=my_file,line=2,title=Missing Coverage::Line XXX is missing coverage

I will get these in the job logs:

And in the Pull request view it will show this:

Which is very neat.

However right now, I need to use diif_cover to create a json and the parse that to print out the annotations. I think it would make sense to have it as an option in diff_cover, but to me it looks like there's no way to inject custom reporting.

I think this could be done either using a hook system like with violations or by letting users specify "--format" or something.

I can contribute the PR, just wanted to hear what way you think it should be implemented