Add an optional `default` argument to trait metadata get, like the Python dict's .get() default argument

After this, trait_metadata('traitname', 'metadata_key', DEFAULT) will return DEFAULT if the metadata key is not set.  DEFAULT defaults to None, like the existing behavior and like the python dict .get() behavior.
Merge pull request #6331 from jasongrout/traitlet-metadata-default

Add an optional `default` argument to trait metadata get, like the Python dict's .get() default argument