Calling Save Images from Python Interface

[KevinJ90825]

Description of Problem & Solution
I would like to be able to generate images for split scenes with the Python API, in the same way you can generate them with the CLI save-images command

Proposed Implementation:
The addition of an API method in the video-splitting module, ideally, or wherever else might be deemed appropriate.

[Breakthrough]

TODO: Add SaveImages command to the Python API.

[Breakthrough]

Implemented global function scenedetect.scene_manager.generate_images in v0.5.2-dev, and still TODO is to add an actual save_images method to the SceneManager which will invoke the generate_images function.

[Breakthrough]

This function will need to be modified either as part of #123 / #125 or another changeset afterwards. If #123 / #125 have to be deferred to v0.6 because they require breaking API changes, this will be implemented first in v0.5.2 (as it is a non-breaking API change).