Better way to support various schema types

In current design, **ramlev** detects schema type by scanning the content of schema, it's not good and accurate if **ramlev** is used to support any kind of schema.

Proposal,
- Make **ramelv** un-aware of specific schema type, let specific schema validator pluggable.
- Provide a CLI options `--schema-type` to specify schema type, which will used to select the underling schema validator, and delegate the validating job to pluggable schema validator