Improve FormatRelative (non-backwardscompatible)

FormatRelative can now be called with a main format and a fallback
argument for more complex formatting of time outputs. For example, one
might expect FormatRelative to figure out that a given time occurs on
next Tuesday at 8:00 AM. Passing it the format string "%s at 15:04"
would produce "Next Tuesday at 08:00", according to package time
formatting and Sprintf.