Add support for VALUE=DATE as a RDATE param

[tchap]

I checked other functions and it seems to me that simply allowing VALUE=DATE will work since the time part is optional in the parsing function.

References #127

[http-teapot]

It'd be great if this PR could be merged.

const r = [
  "EXDATE;VALUE=DATE:20150610",
  "RDATE;VALUE=DATE:20150609,20150611",
  "RRULE:FREQ=DAILY;UNTIL=20150628;INTERVAL=3"
];
rrulestr(r); // unsupported RDATE parm: VALUE=DATE

const r = [
  "EXDATE;VALUE=DATE-TIME:20150610", // using DATE-TIME instead
  "RDATE;VALUE=DATE-TIME:20150609,20150611", // using DATE-TIME instead
  "RRULE:FREQ=DAILY;UNTIL=20150628;INTERVAL=3"
];
rrulestr(r); // succeed, though it appends time to date which isn't critical at all

[espen]

Can you add some tests for this?