added preferred-editor fields to tiddler to allow per tiddler choice of ...

...editor

I've put a demo here 
http://preferrededitor.tiddlyspot.com/

The result doesn't give a great user experience - when you type the name of the editor into the 'preferred-editor' field the first char get saved immediately causing the edit template to change the editor (which give an unknow editor message).
Maybe we can provide a drop down list next to the type list (when there is a preferred-editor field) giving the available editors to choose from?