Add JSON output to the `rbt post` command.
Add JSON output to the `rbt post` command.

This change adds JSON output for `rbt post`.

Testing Done:
- Ran unit tests
- Tested `rbt post --json`

All tests of JSONOutput are included in r#11521

Reviewed at https://reviews.reviewboard.org/r/11560/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools