Supply custom bindings with actual observables

As discussed in issue #2, when using the es5 plugin, bindings aren't provided with the observable, only with the value of the property. 

@rniemeyer suggested an approach to using a custom binding provider to supply bindings with actual observables. This approach should in theory support any existing two-way custom bindings developers may have written for knockout before applying the es5 plugin.

This PR is an attempt at implementing such a provider.