Ember.Handlebars.precompile supports knownHelpers

:fire: @zendesk/browncoats @shajith @chrsjxn

`Handlebars.precompile` supports a second `options` argument. Sadly, `Ember.Handlebars.precompile` did not; it simply defined its own `options` and passed them in. This adds support for passing in that second argument. Specifically, the compiling client can pass in `knownHelpers` so that Handlebars won't use `helperMissing` to do all helper lookups.