Add Spendable convenience fn to tx.Outputs list class
Add Spendable convenience fn to tx.Outputs list class

Makes it easier to iterate over usable outputs with foreach, while still preserving/providing the index of the txout in Outputs[]

bitcoinbrisbane/NBitcoin,dangershony/NStratis,NicolasDorier/NBitcoin,HermanSchoenfeld/NBitcoin,MetacoSA/NBitcoin,MetacoSA/NBitcoin,thepunctuatedhorizon/BrickCoinAlpha.0.0.1,stratisproject/NStratis,lontivero/NBitcoin,you21979/NBitcoin