add function that allows feeTo to claim airdrops

[NoahZinsmeister]

we will probably need to include balance checks to make sure that e.g. tokens which forward to a common implementation from different addresses can't be drained by feeTo

[moodysalem]

alternatively to feeTo being a fixed address on factory that receives fees, could point to a controller that has a method that returns feeTo, which can optionally return a constant address, or a return a different address for each pair