fix(datafile accessor): Add datafile accessor method to TS definitions (#564)

Summary:

Add new datafile accessor method to TS type definitions

Test plan:

Manually tested with npm link and local app

Author: mjc1283

packages/optimizely-sdk/lib/index.d.ts

@@ -257,6 +257,7 @@ declare module '@optimizely/optimizely-sdk' {
       [featureKey: string]: OptimizelyFeature;
     };
     revision: string;
+    getDatafile(): string;
   }
 }