Add option to send MAX amount of coins

[Paouky]

While I was making a simple guide on how to send/receive grins on Niffler, I sent 1 coin to my Niffler wallet. After I was done, I wanted to send this 1 grin back to my original wallet. However, I can't completely spend this output, as Niffler requires me to leave 0.01 for a fee, even though the fee in practice is 0.008. So I ended up leaving 0.002 on my Niffler wallet, which is bloating the UTXO for no reason, as I never plan to use it.

I suggest to add a MAX option which will spend the entire wallet minus the exact fee amount. It would benefit the network by not leaving tiny outputs behind which are never going to be used.