Support using GCS emulators

[manuteleco]

Official Google Cloud Storage libraries usually allow configuring them to perform requests against a GCS emulator server, instead of the real service API.

The user can define a STORAGE_EMULATOR_HOST environment variable pointing to a emulator server. When defined, client libraries will:

• Direct GCS API requests to that host
• Disable (or don't enforce) authentication

It would be great to have support for GCS emulators in googleCloudStorageR as well. This is generally useful in the context of automated tests. And depending on the emulator implementation, it can even give local filesystem support to applications that were previously hardwired to use GCS.

I've already sketched a solution that I have been using for my own purposes in the last few days. I'll open a PR shortly.