Convert to Webpack

[rossjrw]

This PR converts the project structure to a Webpack-based build, which will make future changes much more reliable (if not any easier).

• Use Webpack
• Implement Babel
• Get manifest version from package version
• Add build instructions to README

To build, run npm run build and then the dist/ dir contains everything the extension needs.

Functionality is unchanged. Extension works fine for me in Edge as an unpacked extension.