Indented partial calls are don't result in indented source

[jacobstern]

Per the reference for Handlebars.compile(), indented calls to a partial are expected to result in the entire source block being indented. This does not work with handlebars-idom. I'm not sure whether this is a real use case but it's definitely a deviation from the reference behavior of the library.

In order to implement this we would have to do something mildly adventurous like proxy calls to the IncrementalDOM backend to add indentation.