Added potassium to potatoes and phoshorus to Pumpkins

I have added some potassium to potatoes and phosphorus to Pumpkins to
give traitorous hydroponics some escape options (they can mix smoke with
this + sugar). I have chosen those plants for realistic reasons:
Pumpkins got a lot of phosphorus and potatoes a lot of potassium.
Merge pull request #279 from Tramziller/master

Added potassium to potatoes and phoshorus to Pumpkins