Use native node filesystem integration instead of shelling out

[davemyersworld]

We noticed increasingly regular timeouts when running cypress-match-screenshot in a docker container on drone for any command that interacted with the filesystem. In an effort to improve reliability, we switched the filesystem manipulation to native node integration instead of shelling out, and we've found this to solve the problem.

I believe this would also solve the concerns expressed in #11

There is of course, a pretty big downside of requiring the task to be integrated as well.