Added in ability to open certain screens when triggered by a Mixpanel push notification

This enables the app to open up the Reader, Notifications and Stats from a Mixpanel push notification. In order for this to work the push notification should have the following payload:

```
{ "origin":"mp", "open":"notifications"} // Opens Notifications
{ "origin":"mp", "open":"reader"} // Opens Reader
{ "origin":"mp", "open":"stats"} // Opens Stats

```

cc @mikejohnstn