Add support for JSON output mode

[jamiebrynes7]

It would be very useful if the CLI supported a --json flag to format stdout in a way that scripts/machines can easily parse. At the moment, you have to parse the CLI's output which is quite fragile.

For example:

$ ob sync-list-local --json

Would output something like:

[
	{
		"vault_id": "abdc...",
		"path": "/Users/jamiebrynes7/...",
		"host": "...",
	}
]