Reimplement GitHub style (#32)

Author: tolauwae

src/reporter/Style.ts

@@ -68,4 +68,9 @@ export class Plain implements Style {
         failure: ' FAIL ',
         error: ' ERROR '
     }
-}
+}
+
+export class GitHub extends Plain {
+    bullet = '::group::';
+    end = '::endgroup::';
+}