Add relative to other tests 'run' mark 'after' and 'before' kwargs

[mondeja]

Closes #65

As was reported, relative to other tests module ordering is not working because is not implemented.

[mrbean-bremen]

I actually prepared a PR for the same feature, but had trouble to get it running in the CI (a compatibility issue with pytest-cov) - so you beat me to it :)
It would also make sense to update the documentation (docs/index.rst) to move the related part out of the "Aspirational" chapter.

[mrbean-bremen]

Looks good to me, thanks!
@ftobia - the Travis builds are currently failing due to an incompatibility with the latest version of pytest-cov - I'm looking into this. But they are also not visible on the PR; I had a similar issue recently, still unresolved - maybe you can check the stettings.

[mrbean-bremen]

@mondeja - I didn't notice at the time, but there is already a PR (#37) that implements this. I integrated that in my fork (I was checking the oldest PRs first), so yours didn't get integrated, although I had planned to do so first...

[mondeja]

so yours didn't get integrated, although I had planned to do so first...

Don't worry about that.