Allow third-party packages to register new rbt commands.
rbt will now scan for commands that are registered for

the 'rbtools_commands' entrypoint.

Created a simple Python package that registered a new command, and sure enough, it showed up in rbt help, and could be invoked via rbt [command].
I created a third-party commands that tried to override the built-in "post" command, and tested to make sure that only one "post" command was listed with rbt help, and that only the built-in post command could be invoked.