New Features

• draft 2019-09 support
• detects infinite loop in schemas
• new flag -draft in command jv
• add examples in godoc

Bug Fixes

• eca6e45 sometimes $id getting applied more than once
• 1b9ea47 Compiler.Compile always return *SchemaError

Incompatible Changes

• extension api changed
• if "$schema" missing, it defaults to draft 2019-09 (latest) unless overridden