[html-formfu] new DateTime inflator formatting method

[Carl Franks]

The DateTime inflator now has a strptime() method to set formatting
when the object is stringified.

You can either pass a simple string, such as '%d-%m-%Y'.
Or you can pass a hashref of options that will be passed onto the
DateTime::Format::Strptime constructor.

There's now documentation for both this and the parser() method.

Cheers,
Carl