Support Sphinx features like rst_prolog

[ITler]

Sorry for filling up your issues list. I am not an experienced python coder, so I am kind of wondering, what is possible for m2r2 in regards of supporting Sphinx provided features.

I would like to see for instance the rst_prolog feature to work, because it would help a lot with DRY documentation.

If it is not possible to provide that support or you are not planning so, it is okay. I just would like to get a clear understanding if I can count on m2r2 and build my use-case around it, or whether I should look for another approach.