Add ESLint & Travis CI

[yfdyh000]

Fixes #243.

Travis CI works: https://travis-ci.org/yfdyh000/webextensions-examples/builds

[wbamberg]

This looks great to me, but it's also a significant change to the policy for this code. I'd love it if @rpl and @andymckay could take a look at this and comment on whether this policy, and this particular ESLint configuration, looks good to them.

[andymckay]

I'm no expert on eslint, but this looks like a good start to me and its something we can tweak later if we want to. Thanks @yfdyh000 for doing this, I think it will make the repo better.