Plugin System (Dynamic Detector CLI)

[Breakthrough]

Description of Problem & Solution
Right now one cannot dynamically add detectors to the command line. This prevents having additional detectors be "installed" from other packages, and requires users to run/install PySceneDetect from source if they want to add a new one to the command line.

Proposed Implementation:
See https://packaging.python.org/guides/creating-and-discovering-plugins/ for some possibilities.

Essentially, one should be able to register a detector, which PySceneDetect can then iterate over, and register in the command line when initialized.