(Feature) Use backstop.js instead of backstop.json by default

Hi @garris

Did you ever considered using a backstop.js(by default) instead of the backstop.json.

I know one can use it with `backstop test --docker --config=backstop.js` but I think it would be more efficient for the future.
I use this for all projects where I use backstopjs.

Advantages:
- you can add comments to config file
- blocks and be out-commented
- code can be generated dynamically
- more readable
- zillions of double quotes disappear in the config file

Disadvantages:
- ?

Here is a full example of "kind of" what I mean and how I would use it.
https://github.com/klodoma/backstopjs-debug/tree/feature/js-config