Improve tools for fitting and testing topic models in command line

[mjwestgate]

Currently it is possible (though not straightforward) to take a model from screen_topics and use it in the command line, but not to run a model using run_topic_model and pass it to screen_topics. It would also be useful to have some sort of model fit parameters to allow users to optimise k, then visualise that model with screen_topics. One option for this would be to switch from topicmodels to stm for fitting topic models, though it is unclear what the implications of that switch would be in terms of consistency and dependencies.