Add script to help users run models

[huddlej]

As noted in #48, each new evofr-based analysis ends up needing its own copy of a run-model.py script that accepts a model configuration YAML, instantiates a specific model type, runs the model, and serializes the model results to JSON. It would be helpful for current and future projects if evofr provided a standard command line interface to run models that would avoid the need to copy run-model scripts across projects.

The interface could be a generic script that appears in the user's executable path (e.g., run-evofr-model.py) or it could be part of a simple evofr CLI like evofr run-model in the style of augur, samtools, etc.

As examples, see the MLR model script and renewal model script from the forecasts-ncov repo.