[webextension] — refactor + linting + CI + auto deployment

[ffoodd]

• [ ] Refactor JS to reduce duplication and better use ES6 features
• [ ] Apply ESLint (and Prettier?) → this could imply a totally different workflow: would a separate package.json in the /webextension folder be easier?
• [ ] Whatever is done before, hook it on Travis to check future PR :)
• [ ] And may worth a try to auto deployment.

Here are some resources:

• Mozilla's addon linter,
• which is part of Mozilla's web-ext project, "A command line tool to help build, run, and test web extensions";
• food for thoughts for Chrome webstore autodeployment by Gokul Kathirvel.

[ffoodd]

@rik mentionned the use of async / await to improve readability: https://gist.github.com/rik/e6f63b5557cdf4e34cc5c82da172a6f1