Support for user-defined fields in "st_CachedMenu" tag

Right now we have nice <inp2:st_CachedMenu .../> tag, that prints every menu on website without doing a database query.
This works good, however in some projects I came across situations where a new database field (e.g. extra css class name) was added to a page (category) and this field needs to be used during menu printing.

This requires to override 2 methods in MenuHelper class.

I'm proposing to create an array (as a property of MenuHelper class), where mapping is created between Categories database table fields and parameter names, used to retrieve their values from template.

Here is an example array:

Array (
	'parent_path' => 'ParentPath',
	'icon' => Array ('Icon', 'resize:100x100'),
);

Then these parameters would be used with GetField method.