Basic if helper

A basic implementation of the if helper (implementation & tests borrowed from Handlebars).

I changed the helper signature to be (params, options, env) across the board now to bring it closer in line with bound-templates. All the other properties like context, moroh, element are stashed on options.

A few things:

- Should paths be pre-resolved against the context, like in streamifyArgs?
- The hooks and the helpers are coupled. They should be grouped together under an htmlbars-static package. Suggestions for other names welcome.
- Need a way for the dom helpers to be swapped out without blowing away what's stored in `env`. I changed the template to use Object.create, but there might be a simpler way? I think we should be storing dom in the options hash instead.
Can this be shared across tests in a support file?