Provide last toggled at timestamp for each feature flag

[eggman64]

{
   "name":"amazing_feature",
   "created_at":"2018-03-16T13:38:19Z",
   "status":{
      "env1":true,
      "env2":false
   },
   "feature_status":[
      {
         "name":"env1",
         "status":false,
         "toggled_at":"2018-03-16T13:39:19Z"
      },
      {
         "name":"env2",
         "status":true,
         "toggled_at":"2018-03-16T13:39:19Z"
      }
   ]
}