[html-formfu] file-stem support + better catalyst handling of conf files

[Carl Franks]

I've added a new $form->load_config_filestem() method.

It works just like load_config_file(), except you don't have to
include the file-extension in the given filename(s):

    $form->load_config_file( 'filename.yml' );
    $form->load_config_filestem( 'filename' );

I've also changed Catalyst-Controller-HTML-FormFu, so it uses this new
method - this means you no longer need to set 'config_file_ext', as
it'll automatically find any files with extensions that Config::Any
knows about.

This also means you no longer need to include the file extension in
explicit method-attribute arguments:

    sub my_action : FormConfig('path/to/file') { }

(Although it still works it you do include the file extension)

Carl