Adds JSCS configuration file.

This change introduces a [JSCS](https://github.com/mdevils/node-jscs)
configuration file which allows us to lint the style of any changes
made to our scripts against the Google JavaScript style guide.

I’ve verified that app/main.js currently passes using the “google”
preset. At this time, I think we should only use the file along with an
editor plugin rather than introducing build-time tooling which may
bloat the Gulpfile.  This seems fine for everyday use.

reviewer: @sindresorhus