Add structured output formats to CLI

[jtracey]

Currently, the CLI's output is formatted as some hacked together human-readable strings. You could probably get decently far with piping it into something else if you really wanted to, but it's not intended to be stable or designed to be easy to do so. We should add more structured output, for easier use with other programs:

• JSON: Universally understood, should be pretty simple, especially if we use a library like Serde.
• Raw: Just dumping the objects out as they're read off the wire. In some ways would basically turn the CLI into a sanity-checking netcat, but it could still be useful, especially for debugging and creating tests.
• Whatever else? Again, if we use Serde, it should be pretty easy to support other output formats.