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.
Instead of replicating this logic in both module formatters it'd be best to make `getModuleName` return the built literal and return a falsy if there's none to be inserted.

So instead of

```javascript
if (this.insertModuleId) {
  var moduleName = this.getModuleName();
  defineArgs.unshift(t.literal(moduleName));
}
```

you'd have:

```javascript
var moduleName = this.getModuleName();
if (moduleName) defineArgs.unshift(moduleName);