Add support for django __ syntax to add_related_count rel_field

[ErwinJunge]

This PR adds support for __ syntax to the rel_field argument of the add_related_count method.

The rationale is that sometimes you might want to count objects that are a multistep relation away from the mptt model. I've added a model to the testsuite (SubItem) and associated tests to showcase the idea and test the implementation.

[matthiask]

Thanks!

[ErwinJunge]

@matthiask can you help me out a bit with achieving my hacktoberfest goals and apply a hacktoberfest-accepted label to this PR? Alternatively, if you want to participate in hacktoberfest, you could apply a hacktoberfest topic to the repository :)

[matthiask]

@ErwinJunge Done!

(I received a few spammy pull requests at the beginning of october, so I don't know whether I want to add the label to the repository. )

[ErwinJunge]

Awesome, thanks :)

Sorry to hear about people spamming, I understand that's the reason for the explicit approval process now. Probably better to leave the label off then, it might be more trouble than it's worth.