Export a new function to get the max amount of satoshis that can be sent from a given set of utxos at a given fee and including any miscellaneous target outputs.
Initially I was going to implement this function in Cashtab, but it requires bytecount methods in this lib and I believe it is a common enough thing for a utxo-chain dev to need that it should be in a lib. I considered modifying the coinSelect function to simply sendMax -- however this would make it difficult for Cashtab's needed use case (displaying the value before the user decides to send it) -- which is also a common one for app developers. An app developer can easily enough still use coinSelect when building the actual broadcasting function.