Fix data-overlay example

[yewyewxd]

This PR is a continuation of #22 (should've branched out from the latest commit of original master branch, will improve next time!);

New changes starts from e074830, prior to that, all commits are from #22

Steps taken:

1. Replace version of the dependencies in package.json with the latest ones, delete package-lock.json, run npm install
2. Replace outdated mapbox assembly.min.css and assembly.js in public/index.html
3. Documentation: added instruction of adding a new access token (see live changes here)

[Countdown369]

Looks similar enough to the last one 👍