Add more functionality to findBreakingChanges (#874)

Author: sam-swarr

src/index.js

@@ -357,13 +357,16 @@ export {
 
   // Compares two GraphQLSchemas and detects breaking changes.
   findBreakingChanges,
+  BreakingChangeType,
+  DangerousChangeType,
 
   // Report all deprecated usage within a GraphQL document.
   findDeprecatedUsages,
 } from './utilities';
 
 export type {
   BreakingChange,
+  DangerousChange,
 
   IntrospectionDirective,
   IntrospectionEnumType,