Add `campaigns list` subcommand

Created by: mrnugget

This allows the users to list campaigns and get the campaign IDs they need to use the campaigns add-changesets command.

The default rendering template is {{.ID}}: {{.Name}} but users can specify more fields or get a JSON dump of all of them with the described {{.|json}} pipe:

$ src campaigns list -first 1
Q2FtcGFpZ246NA==: testing the src cli

$ src campaigns list -first 1 -f '{{. |json}}' | jq .
{
  "ID": "Q2FtcGFpZ246NA==",
  "Name": "testing the src cli",
  "Description": "This campaign is here to test the `src` cli tool.",
  "CreatedAt": "2019-09-20T05:49:54Z",
  "UpdatedAt": "2019-09-20T13:07:55Z",
  "Changesets": {
    "Nodes": [
      {
        "ID": "Q2hhbmdlc2V0OjM1",
        "State": "CLOSED",
        "ReviewState": "APPROVED",
        "Repository": {
          "ID": "UmVwb3NpdG9yeTo0Mg==",
          "Name": "github.com/sourcegraph/sourcegraph"
        },
        "ExternalURL": {
          "URL": "https://github.com/sourcegraph/sourcegraph/pull/5003",
          "ServiceType": "github"
        },
        "CreatedAt": "2019-09-20T06:30:20Z",
        "UpdatedAt": "2019-09-20T13:32:04Z"
      },
      {
        "ID": "Q2hhbmdlc2V0OjM5",
        "State": "MERGED",
        "ReviewState": "APPROVED",
        "Repository": {
          "ID": "UmVwb3NpdG9yeTo0Mg==",
          "Name": "github.com/sourcegraph/sourcegraph"
        },
        "ExternalURL": {
          "URL": "https://github.com/sourcegraph/sourcegraph/pull/5005",
          "ServiceType": "github"
        },
        "CreatedAt": "2019-09-20T06:32:14Z",
        "UpdatedAt": "2019-09-20T13:32:04Z"
      }
    ],
    "TotalCount": 2
  }
}