Add available stock quantity field for custom part data

[hkleen]

I think it might be useful to allow specifying an available stock quantity for custom parts. Analog to the pricing and link field there could be a kicost:avail or kicost:My_Weird_Parts:avail field that is used to populate the respective column in the cost spreadsheet.

This would allow to easily check if the available local stock of a custom part is sufficient to produce a given amount of boards.