Add --json flag to format rbt output as json.

Review Request #9245 — Created Oct. 4, 2017 and discarded — Latest diff uploaded

Information

RBTools
release-0.7.x

Reviewers

When running the rbt command from a script, it is difficult to parse
the output log messages in order to get necessary information from the
command that was run.

This change adds a --json flag to all rbt commands to change the
output to json to make command output parsing easier from a script.

The rbt diff command hasn't been changed because diff output is
expected on stdout.

All rbt tests passed.