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 parmeter. 
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 a the template tag pair.
Switch to explicit conditional for 0 passed arguments instead of lazy !$tagdata
Merge pull request #1 from jesstech/master

Make the plugin work as a Text Formatting option