Expose <variable> as a field within foreach bodies

[narenranjit]

See comments on #79

Q: " How do I do something different for the first or last item in a loop. For example I may want to generate:

1,2,3,4,5 (note - no comma on last item)"

A: "The easier solution than that is:

Flow.dom.converters.register('join', function(v){ return v.join(',') });

<div data-f-bind="somearray | join"> </div>

A more generic/ better solution may be to expose the array itself as a variable.

<ul data-f-foreach="somearray"> 
  <li> <%= (index === somearray.length - 1) ? "last" : value %> </li> 
</ul>