CLI arguments are not validated correctly.

[carltongibson]

This is related to #650 (which is about --path).

CLI arguments aren't enforced, or given sensible defaults.

The one I hit is --format. If you don't provide this there is no error until you get to the final "Valid schema" message. At that point you get a totally mysterious KeyError:

(apistar) ~/Desktop $ apistar validate --path schema.yml 
Traceback (most recent call last):
...
  File ".../apistar/cli.py", line 163, in validate
    }[format]
KeyError: None

Cracking open cli.py, format there is the missing CLI argument. Sending --format=openapi resolves the issue.

format should be required, so as to provide a helpful error message.
Maybe it should default to openapi...