simulates a crash when launched with url

4.2 shipped without Crashlytics functioning.  this PR adds a way to crash the app by hand but in a way that a user would (hopefully) never accidentally do it.

to test, 
- add `DEBUGGING_KEY=omgwtfbbq` to *~/.wpcom_app_credentials*
- load the app with the following URL: wordpress://debugging?type=crashlytics_crash&key=omgwtfbbq

cc @sendhil