Server Variables

Currently, [OpenAPI 3 server variables](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#serverVariableObject) are not handled at all. They are required to have defaults, so at the very least we should string replace the template values with the defaults.

A better implementation would also expose these as optional parameters for the generated CLI. One complication is that different servers might have different variables, so the ones that get exposed might depend on the current server index.