Call functions *with arguments* from within a template

[tbranyen]

@carldanley any thoughts on the following idea:

template:

{{ obj.toUpper 'test' }}

Would then call the respective function with the passed argument:

template.render({
  toUpper: function(str) {
    return str.toUpperCase();
  }
});

I'm thinking this could be useful for Backbone models:

{{ model.get 'someAttr' }}

[tbranyen]

Closed via #43