Add transaction/output templates for easily creating txs/outputs for custom tokens

[pedroferreira1]

Initial PR discussion: #110 (comment)

We've decided to keep the Output class as we have in the output definition. But we should have an easier interface for the user to create a transaction with custom outputs.

Maybe have a new method/class to create outputs with parameters like tokenUid, isMint, isMelt and this method will translate to our Output object with correct tokenData and tx tokens array.