[enh] add support for multiple outputs

Allows Silkaj to make a transaction to multiple receivers using a new separator in --output option:

--output=<public key>[:checksum][,<public key>[:checksum]]

So the transaction's --amount is sent to each output, which means if we send --amount 10 with 3 outputs we actually send 30.

I've tried to integrate it without many modifications as I don't know the possible impacts elsewhere.