Define output amount in swap

#### What would you like to be added:
I'd like to be able to swap an undefined amount of DFI to a defined output of dUSDC.

For example I want 500 dUSDC for some DFI. At the moment I can only define the input amount of DFI to get ~500 dUSDC, but cause of slippage and price impact it will never be exactly 500 dUSDC.

I'd like to have an option that the slippage will be used for the input instead of the output.

#### Why is this needed:

- I want to payout an exact amount of dUSDC or an other coin which I don't own in this amount yet.
- I want to save a specific amount of USD for taxes.