Add default session to jbrowse_config.json

[teresam856]

Proposal: Would it be helpful to add a default session so that when someone initially clicks on http://localhost:3000/?config=http://localhost:9000/jbrowse_config.json, it takes them straight to the HelloView example? Just wondering if this would help some developers, instead of having to navigate to File -> Add -> OpenHello!

  "plugins": [
    {
      "name": "CustomizedCorePlugin",
      "url": "http://localhost:9000/dist/jbrowse-plugin-customized-core-plugin.umd.development.js"
    }
  ],
  "defaultSession": {
    "name": "Integration test example",
    "views": [
      {
        "id": "integration_test",
        "type": "HelloView"
      }
    ]
  }
}