Support multi-sig transactions

[ethanfrey]

In theory we do, now in practice.

Add a --prepare=<file> flag that is honored by all tx commands, and outputs the tx as json after signing it.

Add a --multi option to all tx commands so they wrap the tx with a multisig wrapper not a single sig wrapper. Very important with --prepare

basecli tx (no subcommand) will take --in=<file> to load an existing tx. You can then use --name=<signer> to add another signature, and either post it directly, or --prepare it into a second file to collect even more sigs.

This may not be the most user friendly, but the functionality is all there and we can add better UX when we actually have experience using it.

[ethanfrey]

This was dealt with in #176