Support multiple include paths

[lu-zero]

I'm evaluating docurium for Libav and seems that it doesn't support multiple include paths (ideally I'd have ["libavcodec", "libavformat", ...] and such as input.

[jameswalmsley]

I did fork docurium a long time a go (when it seemed like the project was no longer active) to add such a feature. Now I've seen that docurium is active again I shall back-port my features to it and make a pull-request.