Add hook to call on missing properties

Before filing issues, please check the following points first:

• Have a look at https://github.com/wycats/handlebars.js/blob/master/CONTRIBUTING.md
• Read the FAQ at https://github.com/wycats/handlebars.js/blob/master/FAQ.md
• Use the jsfiddle-template at https://jsfiddle.net/9D88g/47/ to reproduce problems or bugs

Check out the jsfiddle at https://jsfiddle.net/fwrww8tj. What is demonstrated there is this problem: when registering a helper for catching missing keys in the data using:

Handlebars.registerHelper("helperMissing", function(context, options) { ... })

the helper function is called correctly when keys are missing on the top-level, but fails when the key is down the path in the data somewhere. Consider the following data set:

{
    foo: "foo",
    nested_property: {
        foo: "foo"
    }
}

When the template calls for {{bar}}, the function is correctly called. However, when the template calls for {{nested_property.bar}}, the helper is NOT called.

Start with the jsfiddle reproduction and the Handlebars.registerHelper("helperMissing", ...) entry point, comparing top-level {{bar}} with nested {{nested_property.bar}}. Trace how missing properties are resolved and add a regression test showing the expected hook behavior for nested paths; done means the test passes and the reproduction behaves consistently.