Support loading task definition from local JSON file

[psyhomb]

It would be nice to support loading task definition from local JSON file.

[bfox1793]

+1 to this feature. There are some smaller things that ecs deploy can't handle when updating a task definition, and while it'd be great if it was all available via simplified command args at all times, this would remove the need to wait for feature updates to ecs deploy (as the task definition json will follow whatever AWS standards are at the given time, auto-supporting any new features)