use the default externs when compiling code with the closure compiler.

I found [this thread](https://groups.google.com/forum/#!topic/requirejs/_JelcAq8j64) about r.js not including the default externs when compiling using the closure compiler in the ADVANCED_OPTIMIZATIONS mode. I see you were involved but, unless I'm missing something, it looks like you never added the functionality to r.js.

I created a small patch that does the trick, it's just a few lines so I hope is small enough for being pulled without me having to sign any CLA.

On a side note, the command line compiler has a --use_only_custom_externs parameter to prevent the default externs from being used. Such parameter is not available in the [CompilerOptions](http://closure-compiler.googlecode.com/svn/trunk/javadoc/com/google/javascript/jscomp/CompilerOptions.html) class so I guess it should be implemented from the outside. I preferred to avoid to add a new configuration option, let me know what you think.