add rewrite transform + quote some text directives

[dmachard]

This PR implements #527

• add rewrite transform
• add tests
• quote dnstap version directive
• quote dnstap identity directive
• quote peer-name
• add docs
• Update README.md

Config example

  - name: tap
    dnstap:
      listen-ip: 0.0.0.0
      listen-port: 6000
    transforms:
      rewrite:
        identifiers:
          dnstap.version: ""
          dnstap.identity: "foo"