Ability to add key file using environment variable

[bnap00]

Currently this package is accepting path to GCS key file. We can add an option where user can add the content of key file to environment variable. This can help while using this package for open source application and when one does not want to store the key file in git or server disk.

[ludalex]

https://github.com/ARozar/multer-google-storage/pull/14

[AnthonyMouallem]

+1

[woojunro]

Any updates?