Support for converting ANNIVERSARY

vcard 3 -> 4
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Find all X-ABDATE and X-ANNIVERSARY properties, and merge the ones with identical values. Convert all of these to ANNIVERSARY.

vcard 4 -> 3
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Convert ANNIVERSARY to both X-ABDATE and X-ANNIVERSARY.
don't we usually use php 5.4 array notation?

`list($input, $output) = [$output, $input];`