Added support for JavaScript third-party debuggers

Summary:* Add ability to configure the app that should open when starting debugging

axemclion discussed this feature with tadeuzagallo and martinbigio on: https://github.com/facebook/react-native/issues/5051
Closes https://github.com/facebook/react-native/pull/5683

Reviewed By: martinbigio

Differential Revision: D2971497

Pulled By: mkonicek

fb-gh-sync-id: 91c3ce68feed989658124bb96cb61d03dd032599
fbshipit-source-id: 91c3ce68feed989658124bb96cb61d03dd032599