Add parsers to the "Reference" page

[bundfussr]

It is hard to find out which parsers are available. For example, parse_cdisc_datetime() is mentioned in the Changelog but not on the as.parttime() reference page. I would suggest to add a "Parsers" section to the "Reference" page and link to this section from the format argument of as.datetime().

[dgkf]

Great idea! Will definitely add a section specifically for parser options.