Add support for goog.module based imports

- option to enable goog modules
- golden file based tests
- based on moduleName for the current file

Based on @vojtajina's initial work.
I think we cannot support that at the moment.
- Closure Compiler needs to statically analyze the export/import relationships. That means we cannot just `export = merge({}, starFromModA, starFromModB, ...)`.
- To statically enumerate the list of symbols from the other module, we'd need to look at other files during transpilation, which we don't want to do for performance reasons AFAIU

So without Closure Compiler growing support for this, it won't happen. I don't really consider the feature critical though.

I'd have to change the base class a wee bit to properly report an error, do you think that's worth it? As is, you'd end up with an error from Closure Compiler later down the line.