arcyon task-create: support '--format-id'

Change the default output of 'task-create' to match the style of
'create-revision', i.e. user-friendly output by default and
machine-friendly if requested.

Test Plan:
$ ./precommit.sh
arcyon: support 'format-url' for some commands

When creating or updating revisions and tasks, it may be useful to print
out just the url of the new/updated item for consumption by wrapper
scripts.

Added to
- arcyon create-revision
- arcyon update-revision
- arcyon task-create

Test Plan:
$ ./precommit.sh
Try each of the --format-* options for the affected commands