Option to create eth transaction file(s) that can be executed separately

Right now, modify commands send a transaction directly to the Ethereum network. There should be an option to just output a file containing an unsigned or signed transactions, which can then be sent to Ethereum by a different tool.

There is already such a format that is used in Urbit: https://github.com/urbit/urbit/blob/master/pkg/arvo/mar/eth/txs.hoon