Avoid writing to node_modules/backstopjs/

I noticed that backstopjs is creating temporary files such as `config.json` in `node_modules/backstopjs/config.json`. I would like to suggest to use `os.tmpdir` instead.

My case might be special in that my `node_modules` folder is actually read-only but I really think that `node_modules` should only ever be modified by `npm` and not by random packages storing temporary data there. It does not seem like an idiomatic behavior to me either - I haven't had this issue with any other package so far.

Would there be any objections against storing temporary data in directory below `os.tmpdir()` ?