Added option subTxt to search in subelement

I needed to search in a subelement of my li instead of simple li text
So I added an option 'subTxt' which get a selector for children of li where we search
For example, if li contains a span with .info class, you specify {subTxt: '.info'} in options in order to filter with this span content text
Useful for complex li content (but surely slower because of jQuery find() function)