Added some stats for twitter deep linking.

- Added super property “reader_opened_from_external_url”
- Added super property “reader_opened_from_external_url_count”
- Added event “Reader - Opened From External Source” (to help with
  funnels)
Making tweaks based on @aerych’s feedback.
Merge pull request #914 from wordpress-mobile/issues/913-add-mixpanel-stats-for-twitter-deep-linking

Added some stats for twitter deep linking.