There should be a configuration option to throw if a tag finds no match in the source

Given data with a missing property represented in the source, it should be configurable to have this throw.

```
var source = 'Some text {{foo}} with {{missing}} tag';
var template = Handlebars.compile(source);
var data = { foo : 'bar'};

//should optionally throw
var result = template(data); 

```
