Make AMD module id's optional

Here's a PR for #186.

Since I really think inserting module id's should be _opt-in_ rather than the default (i.e. anonymous modules should be the default output), I'd like to get your opinion on this first.

Then either I need to fix the tests, or the default setting for this feature.
You can access the options from `this.file.opts` from inside a module formatter so this isn't necessary.