Farming Update, take 3

Add an attribute "grow" to seeds to allow variable plant growth times.

Tested and it compiles. I'm really hoping to get this merged this time, and I'll do whatever needs to be done to fix it.
This is not loaded anywhere. You need to load it in item_factory.cpp, search for where `fun` or any of the other members of the comestible type are loaded and add it there. And please document this in doc/JSON_INFO.md (there is already a section for comestibles, just add it there and explain what the value means).

Btw. why 91? This looks like an arbitrary number why have you chosen this one? It might e a bit puzzling for moders.