add optionsAfterRender binding parameter for the options binding

This works similarly to the `afterRender` option for `foreach`.

My main motivation for adding this is to support arbitrary bindings for the select options as described in #382. If we agree to include this change, it may be sufficient for most people who want more control over the `option` elements. 

This change will also make it possible to support an `optionsBind` option using a simple plugin.