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.
@domenic that is interesting, but it has a bunch of limitations - e.g. I need to set the source name per file during compilation, and baseline-tester only supports a `function(string): string`. Given that the golden file test code is not that complex here, I think it's easier to just keep it this way.