Add create param to fieldmangler

This code adds a create param to the fieldmangler widget. It uses standard .tid file syntax and the deserielizer for x-tiddler. I tested this with a macro, but I don't know if it works in the tag declaration.
```
\define createStateTiddler()
tags: $:/tags/stylesheet

$(default)$
\end
<$fieldmangler tiddler="$:/temp/test" create=<<createStateTiddler>>></$fieldmangler>. 
```
Here are two example tiddlers: https://gist.github.com/Arlen22/9752130