Create new commands for input/output encode/decode. #299

The tx-encode command is overly complex due to the overloading of the --output option, supporting three distinct 2/3 tuple modes. The --input option is simpler, but requires an encoded script and index tuple. The input-set command constructs an input and sets it, but doesn't use the input construction model that is used by tx-encode.

To resolve, create commands:

• input-encode
• input-decode
• output-encode
• output-decode

and modify input-set and tx-encode to accept opaque/base16-encoded input and output parameters.