[html-formfu] new config_callback - formfu + cat controller

[Carl Franks]

HTML::FormFu has a new config_callback() method, which accepts args
suitable for Data::Visitor::Callback, and are used by
load_config_file() to preprocess any config files, before passing
their data to populate().

The Cat controller has a new option config_callback(1), which when
true sets up a callback which searches for any instance of
__uri_for(URI)__ in the config file, and replaces it with the result
of $context->uri_for(URI).
This is ideal for setting the form action from the config file.

This is on by default - see the docs for further details.

Carl