rename to indicate that this plugin only sync's `variables`

[doapp-ryanp]

I think the name of this repo should be changed to indicate that this plugin only sync's variables in the _meta directory. As it is now, its misleading.

IMO syncing things not related to variables in the _meta dir would be a mistake, as that stuff needs to be in version control to facilitate team development. See serverless/serverless#928 for more background on why I feel this way.

Few brainstorming ideas for name:

serverless-meta-var-sync
serverless-var-sync
serverless-variables-sync