core-datepicker: Support empty/no value

[skjalgepalg]

Use type="button" in example code in docs Add handler for filling empty input when clicking todays date.

Relates to #633

[skjalgepalg]

This PR adds the ability to initialise a datepicker without a value.

• [Breaking] The timestamp-attribute has been removed
  • Note: Limited support is kept help aid with migration; value is treated like date (see below) but logs to console.warn on initialisation to help identify where migration is necessary.
• New optional date-attribute takes any value supported by simple-date-parse
  • if date is set to null the attribute is treated the same as if removed through removeAttribute.

When date has no value:

• All getters return null
• Selecting the current date will dispatch a datepicker.change event
• If a table is drawn:
  • the current month is drawn
  • autofocus and tabindex are set to the current date
  • Keyboard navigation (up/down/left/right arrow) start from the current date.

The following has been fixed:

• Support for external elements using data-for (and deprecated for) -attribute

[olefjaerestad]

Left a couple of comments on the code!

As an aside: The nrk.no team is currently using core-datepicker in at least a couple of apps: https://github.com/nrkno/nrkno-sendeplan and https://github.com/nrkno/nrkno-sportsopplevelse. These apps are currently not in use, as they've been season/event specific so far. When we start using them again, we'll most likely have to make tweaks to the code anyway, so small changes to the datepicker API (e.g. timestamp being replaced by date) should be no problem :)