Enable exact objects by default in the project template

Summary:
This template for new projects isn't configured with exact objects by default, which could cause issues with code in react-native assuming this option is enabled.

Changelog: [Changed] Enabled exact_by_default in Flow for new projects using the template

Reviewed By: cpojer

Differential Revision: D22571745

fbshipit-source-id: da5affe903114484384764be2142e1c46244bfac

Author: rubennorte

template/_flowconfig

@@ -30,6 +30,8 @@ emoji=true
 esproposal.optional_chaining=enable
 esproposal.nullish_coalescing=enable
 
+exact_by_default=true
+
 module.file_ext=.js
 module.file_ext=.json
 module.file_ext=.ios.js