Support externally specified schemas for validating extensions

[skynavga]

Add support for --external-schema option to permit adding external schema for validating foreign vocabulary. The value of this option should be a pair, NAMESPACE URL, where NAMESPACE specifies the external schema's namespace, and URL specifies the URL locating the schema. If URL is relative, then should be interpreted as a local file path relative to the current working directory. Multiple options should be permitted in order to add multiple schemas.

For example,

--external-schema http://external.com/schemas/foo http://external.com/schemas/foo.xsd

[skynavga]

Fixed in SHA: eb53c29.