WIP: add a JS compare script
This is a generalization of a script developed by @virginia.demilio, Isobel, and potentially others for the Bilby GWTC1 plots. I've made it so one can run on two arbitrary result files. I believe PESummary is the natural place for it.
There is some tinkering left to be done and the docstrings could do with some work, but I'm posting it now to get any input people want to make.