Adds dottime (as defined on https://dotti.me) as a timestamp format.
This format is designed to simplify working with timestamps across
many different timezones by keeping the timestamp format itself in
UTC (and indicating this with a dot character), but appending the
local offset.
This is implemented as a new format because the timestamp needs to be
rendered both as UTC and including the offset, an implementation using
a strftime formatting string is not sufficient.
