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.
Sounds reasonable, though that will lose some of the testing amd and commonjs do - they currently execute the result code, this just compares the generated results.

I think you can argue for just comparing syntax without executing - it might make easier to understand test diffs, and we really only want to make sure we're generating the right syntax, not how the module system works.

Maybe this is something for a later PR?