Bref-local should accept data from --path without Serverless framework

It would be a real nice to have if those of us building Bref on AWS CDK or Terraform could pass a `--path` flag that points to a local file containing our event data rather than having to pass data as a sting argument on each local invocation.

I have opened #1627 to address this and I'm happy to take any feedback on the change request.

Thanks