NPM packge + formatHumanDate

I know it's 2 major changes in 1 pull request, if you prefer I can split it (just got lazy).

1. Adds a setting to the plugin: `formatHumanDate`. It's a callback the user can override to customize the human-formatted date displayed in the pop-over. It would need documentation on your website, I can write it if you need.
2. Adds a `package.json` for npm. This allows to publish the plugin on the npm repository and to create distribution files in a single command (`npm run dist`), useful for contributors.

I hope you'll find all of that useful!