Preserve multiple values with the same key

[marhop]

Hi,

This PR deals with multiple occurences of the same key. Consider the following example input, it has several values with the EXDATE key (I think it's from a Google calendar export):

BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART;VALUE=DATE:20191010
DTEND;VALUE=DATE:20191011
RRULE:FREQ=WEEKLY;WKST=TU;UNTIL=20201007
EXDATE;VALUE=DATE:20200102
EXDATE;VALUE=DATE:20200702
EXDATE;VALUE=DATE:20200709
SUMMARY:Recurring event
END:VEVENT
END:VCALENDAR

Currently, only the last EXDATE item is preserved in the output (slightly formatted for readability):

EXDATE   SUMMARY         RRULE                              DTSTART    DTEND
20200709 Recurring event FREQ=WEEKLY;WKST=TU;UNTIL=20201007 2019-10-10 2019-10-11

With this PR applied all EXDATE items are preserved in a comma-separated list:

EXDATE                     SUMMARY         RRULE                              DTSTART    DTEND
20200102,20200702,20200709 Recurring event FREQ=WEEKLY;WKST=TU;UNTIL=20201007 2019-10-10 2019-10-11

I'm happy if you find this useful, but feel free to modify or reject it! I know next to nothing about awk programming. 😄

Cheers,
Martin