Add support for rinohtype

[brechtm]

Some investigation was done toward adding support for the rinoh (PDF) Sphinx builder. In support of anyone wanting to pursue this goal in the future, I'll link to the relevant issue here: brechtm/rinohtype#155.

[alexfargus]

I'm following up on this from the rinohtype end where I have manged to get a proof of concept working brechtm/rinohtype/issues/272.

I am now looking for a home for my plantuml specific changes. The approach that I have taken is to use a SphinxPostTransform object to convert the plantuml nodes into standard docutils image nodes. See example.

This avoids needing to import any rinohtype specific classes, and I believe should be a generic approach for any currently unsupported builder. However it is a significantly different approach to the current implementation in sphinxcontrib-plantuml. With this is mind, and before I spend significant effort, I would like to know wherther a PR adding support in this way be welcomed here or should I look elsewhere?

[yuja]

@alexfargus I'm not an expert of sphinx, but your approach looks way nicer than the current node substitution logic, which is based off the graphviz extension in sphinx 0.9 era.
I'm happy to take patches that modernize the current codebase.

If you can, please start with a few small commits that just fixes rinohtype support by using the modern API. I'm not willing to review a single big commit that refactors everything. Thanks.