Add a minimum version specifier to the file format.

Taking a leaf from docker-compose, add a top-level "version" field which specifies the minimum version required for the logan file format. The configuration version is different to the logan tool version and should follow the usual semver-style conventions.

To begin with, add a new version, "1.0" so that logan configs can have

version: "1.0"

at the top level and require a version of logan which understands the 1.0 format.