Add support for segments as dimensions

This adds support for using GA's segments as dimensions on the report.

Example usage:

If you want to use the ga:segment dimension, you must specify the segment IDs in your reports.json stream / report config:

[
  {
    "name": "acquisition",
    "dimensions": [
      "ga:date",
      "ga:segment",
      "ga:channelGrouping"
    ],
    "metrics": [
      "ga:users",
      "ga:newUsers",
      "ga:sessions"
    ],
    "segments": [
      "gaid::-1",
      "gaid::U7LSsrWRTq6JIIS8G8brrQ"
    ]
  }
]

Segment IDs can be found with the GA Query explorer. The account configured for authentication must either own the segment, or have "Collaborate" access to the GA view as well as the segment itself having its Segment Availability set to "Collaborators and I can apply/edit Segment in this View".