Implement transform directive for type = base64-line-delimited

This merge request did the following:

• Implement transform directive for type = base64-line-delimited
• A fixture rules file to demonstrate the transform directive, it looks like:

  [
    {
      "match": {
        "type": "path",
        "regex_value":"^/-/test-routable-token/(?<payload>.*)$"
      },
      "transform": {
        "type": "base64-line-delimited",
        "input": "${payload}",
        "output": "decoded"
      },
      "action": "classify",
      "classify": {
        "type": "SESSION_PREFIX",
        "value": "cell-${decoded.c}"
      }
    }
  ]

• Add an integration test for the above fixture rules file.
• Add unit tests for Rule#interpolateValue and Rule#transformMatchResult which are the core for transform logic.

Closes #70