Add tool for backwards calculator

[himynameisdave]

It would be nice if users could work backwards in their calculations. For example, if users knew how much they had for a downpayment, they should be able to enter that and see what kind of asking price they can afford.

M / P = AP

# M = Amount the user has for a downpayment
# P = Percentage multiplier 
# AP = Asking Price

Perhaps it could be a slider, where the user can slide it over based on how much money they have to spend.