Enabled passing in helpers and partials to Mustache views.

Nice for AMD and avoiding globals. Compatible with standard Mustache/Handlebars semantics.

Usage:
```
    renderer(data,{
      helpers: {
         cap: function(s) { return s.toUpperCase(); }
      },
      partials: {
         partial: anotherRenderer
      }
    });
```

Partials can be the view function returned by steal, a `new can.Mustache`, or a plain old function that takes data and returns a string.