v0.8.1

Fixes

• Handle escaped paths in data writes (#695)
• Rewrite with modifiers to allow refs as values (#701)

Miscellaneous

• Add Kafka authorization tutorial
• Add URL query encoding built-ins
• Add runtime API to register plugins
• Update eval subcommand to support multiple files or directories (thanks @devenney)
• Update Terraform tutorial for OPA v0.8
• Fix bug in topdown query ID generation