Draft: yaml: provide command line option to run a schema validation on the given file

It is nice to be able to validate a yaml file conforms to a schema at the command line.

It takes two arguments: --yaml and --schema; both can be either a file path or URL.

There is no output if the yaml is valid. If the yaml is not valid or either of the input files are empty then an exception is raised.

Signed-off-by: Patrick Talbert ptalbert@redhat.com