Establish naming convention for provider plugins

[pmuens]

It would be great to have a kind of naming convention for provider plugins so that one knows what the npm package does / what the GitHub repo is.

Here are my proposals for this:

• serverless-provider-google (my favorite as the provider name in Serverless is google as well)
• serverless-provider-google-cloud-functions

[jthomas]

:+1: on having the provider id match the repo id.