Set up Circle-CI to run linting and build add-on per each PR

Currently it looks like we have a few ESLint errors. It'd be nice to have any PRs fail unless the linters pass and the add-on builds successfully.

I can submit a slightly tweaked ESLint config file shortly, but will probably hold off on auto-fixing errors until after we ship the Shield study, since the PR would add a lot of last minute noise of "var => let => const" and indenting, etc.