Getting to a Web Component

[jekuer]

In order to get closer to a smooth web component, let's work on the following:

1. move the css into the js file (for better encapsuling when using it the vanilla way without npm). But maybe only for the npm version (therefore, this would be a job for the build process to include the css into the JS). And maybe as a variation, so the developer can decide whether he want to import the whole thing or only the js (adding the css as a second step as it is at the moment).
2. extend the HTMLElement with an AddToCalendarButton element. As the new default way. Should still be compatible with the div implementation, but have its own semantic one for future implementations.
3. Offer the option to move the config into data-attributes rather than the JSON way. While the JSON way is still the way to go for Schema.org implementation, data-attributes should be the future default (while still supporting the JSON).