Automate releases

[eejdoowad]

This is a long term idea. Time is probably better spent on features and bug fixes.

Write a release script that:

• is executed with npm run release
• asks for a version number
• asks for release notes
• updates the version number in the manifest
• creates a commit and an accompanying tag
• pushes the commit and tag to github
• triggers a travis build that generates versioned zip-files and creates a github release
• uploads the release to the Firefox Marketplace and Chrome Webstore