A way to let users define custom made bound {{if}} statements

This pull requests makes it possible to create bound {{if}} statements more easily.

Here's a use case:

I'm working on a project where I'm displaying amount of rain that fell within the last hour.
These rainmeters are very fickle however, and don't always send their data to my server.
So when the server doesn't have any data it sends back: {rainfall: null}

Now when the users rainmeter doesn't send data I want the user to know so I show a message:

``` handlebars
{{if rainmeter.rainfall}}




Sometimes the weather outside