refactor plot scripts

[elray1]

each model currently has its own plot script, but much of the logic is duplicated in those scripts. it would be good to clean this up. in a first pass, we could probably put a single plot script into a util folder which accepts some arguments like which plots to create and where to find forecast files to plot. In a second step, we might move some logic over to https://github.com/reichlab/idforecastutils

[matthewcornell]

Good idea. This would be where we'd wrap try() around readr::read_csv() calls to capture 404 errors per #22 .