Make the plugin work as a Text Formatting option

When using a plugin as a custom field "Text Formatting" option, EE passes the field's content to the plugin as a parameter.

This adds that parameter and uses it if it exists, allowing a custom field to automatically run its content through the plugin without the need for the template tag pair.

Note that using a formatted field as a search parameter will only match the field's original, full text and not its url-title formatted text.