Explain usage of non-default built-in VMAF models

[slhck]

Currently it's hard to know how to use a different VMAF model if it's bundled with the library.

This should be done:

• Check the CLI argument for --model-path
• If it is full path that actually exists on the system, use that
• If it is just a string with a valid model name, use the one supplied with the repo
• If it's neither, use the default model
• Rework the default model path function to allow different model paths

Furthermore, improve the documentation to tell users that they can use one of the supplied model names. Perhaps even as a choices setting?