Improve whitespace handling in Time::Format

[HertzDevil]

• Adds support for %n and %t. They are exactly equivalent to \n and \t, as in POSIX C strptime (when _XOPEN_SOURCE is defined) and Ruby Time.strptime.
• All ASCII whitespace characters in the pattern now consume zero or more ASCII whitespace characters when parsing. This includes %n, %t, and the embedded spaces in %c and %r too.
• The YAML timestamp specification only allows the space character and the tab character ([ \t]+), any other whitespace is now disallowed.