Improved `storeParam`

- parse 'params'
- all repeated keys are joined to an array. Sample from google's ical:
  ```css
    BEGIN:VEVENT
    ...

    ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN=Alex Bar;X-NUM-GUESTS=2:mailto:alex@baz.de

    ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=ACCEPTED;RSVP=TRUE;CN=Roman Baz;X-NUM-GUESTS=2:mailto:roman@baz.de

    END:VEVENT
   ```