Add assign colonist to bills

[cheetah2003]

When you make a bill in a particular crafting station, it would be nice to have it be able to limit to only certain colonists.  So like, you could assign your best crafter to make tailored items, while your lesser crafters could be restricted to just cutting stone or making weapons for practice.  The bill interface seems the best place to put this so each crafting bill can be assigned to specific colonists.

[Cazakatari]

Tynan has confirmed that he's working on this problem, for now this is on the changelog for next release:

You can now set a minimum skill level on bills.

I think that will be good enough for me at least