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.
Hm, I preferred `name` since it sounded a bit better than `id` but since most specs use the latter I'm fine with it. The flag should be `--amd-module-ids` though to make it more inline with the other flags `--source-maps`, `--comments` etc.