Feature request: get effective json schemas

Currently there are two JSON schemas available: configuration and elements. Plugins can modify the element schema and #77 (closed) would allow rules to modify configuration schema.

For IDE support it would be useful to expose functions to get the effective schema for a given file.

Suggested solution

Add two new methods to HtmlValidate:

• getConfigurationSchema(filename: string): object
• getElementSchema(filename: string): object