[Feature Req] Support for loading external components via CLI/TOML

[steven-murray]

Is your feature request related to a problem? Please describe.
If using the Python interpreter, externally-defined models can be used. However, these are not available if using
the CLI, or loading from TOML. This should be added.

Describe the solution you'd like
Probably best to convert all base components into ABCs that can register submodules, and provide a keyword for importing modules in the TOML. Then registered plugins can be automatically used.

Describe alternatives you've considered
Could just use importlib directly to load in any string that is given. But this is clunkier and prone to error.