Provide a plugin-based command line mechanism

While the beat commandline is nice, @andre.anjos pointed out that a plugin-based commandline would be more useful. I could not find an issue for this, so I'm creating this but feel free to close it if it is not relevant.

This was carefully designed and implemented in bob:

• bob/bob.extension#44 (closed)
• bob/bob.extension!64 (merged)
• https://www.idiap.ch/software/bob/docs/bob/docs/master/bob.extension/doc/framework.html#unified-command-line-mechanism

A similar approach could be taken here as well.