* BREAKING CHANGE: the library is now stricter about rejecting some invalid
  formats such as "Wed, 22 May 2019 12:00:00 +1 day" (which is a valid date
  spec in some tools but not in ConvertibleTimestamp which does not accept
  relative date modifiers) or "Wed, 22 May 2019 12:00:00 A potato" (where
  the trailing nonsense got silently ignored before this change).
* Time zones are handled more consistently and more correctly.
* Fix some bugs certain formats had with pre-Unix-epoch dates.
* Relax ISO 8601 syntax: allow space instead of T
* Improve ISO 8601 syntax compliance: accept comma as decimal separator,
  accept non-Z timezones.
* ConvertibleTimestamp::convert can take a DateTime now.