Proposal: AMD dependency customization

This Pull Request is intended to open a discussion, not necessarily be merged as-is. If you're in favor, I'll add documentation (and possibly tests?).

What would you think of allowing `options.amd` to be a boolean _or_ a string? If it's a string, we'd use that string as the dependency name, and if it's a boolean we'd use `'ember'` as before. This maintains backwards compatibility, while allowing people to customize what Ember.js's AMD ID is in their project.