Implement --json streaming metadata output mode

[cassaundra]

Outputs events as line-delimited JSON objects to be consumed by something else.

Example:

{"type": "task_started", "task": "build", "timestamp": "..."}
{"type": "task_completed", "task": "build-deps", "result": "skipped", "timestamp": "..."}
{"type": "task_completed", "task": "build", "result": "success", "timestamp": "..."}

Output from tasks should either be directed to a log or perhaps a different stream (e.g. JSON on stdout, task output on stderr so that it can be inspected when only redirecting stdout).

Note: this is distinct from the inspect subcommand, which already exists and dumps information about tasks without running them.