[Feature Request] Annotations

Hello Zathura Team!

First of all, thank you very much for this great product you tailor! It is by far my most preferred pdf viewer (also a vim lover), I've been using it for years now.

The only feature I miss in zathura is the possibility of creating annotations, it really breaks the workflow to switch from one viewer to another just to annotate something and come back to zathura. I could read everything on any other pdf viewer, but they all fail not being like zathura... :(.

Before filling this issue I've carefully looked at the code of both mupdf and poppler plugins in the hope of finding a suitable way of adding this feature and implement it myself. In the end I recognized it should be added at the core level, so I went straight to the zathura code and saw a feature/annotations branch, but also, that is really out of my possibilities to implement it myself.

Best regards, Guillermo.