Added render callback argument

This PR modifies the `$.fn.render` function to add a third `callback` argument. When a function is passed as the third argument, the function is executed in the rendered element's context upon successful application of the template. This is useful in chaining template loads.

The `$.when( /* ... */ ).done()` can also be used to arrive to the desired effect, but this is more intuitive for beginners and for rapid prototyping with lesser codes.

Lint checked and README updated.