Add '--from' and '--to' options to executable + add tests

[jacobperron]

• Add options for overriding 'from' and 'to' domain IDs from the CLI (7e14198)
• Add launch tests for executable (2d2171e)
• Add example config (also validated by tests) (3902e3d)
• Updated README (9cfc223)

Depends on https://github.com/ros2/domain_bridge/pull/6

[jacobperron]

I had to rebase. Then added ab41672. I think this is ready to go.