define "string map" type for new Anti-Features explanations

closes #683

tests/metadata-rewrite-yml/app.with.special.build.params.yml

@@ -1,5 +1,5 @@
 AntiFeatures:
-  - UpstreamNonFree
+    UpstreamNonFree: {}
 Categories:
   - System
 License: GPL-3.0-only
@@ -77,6 +77,9 @@ Builds:
     maven: yes@..
     srclibs:
       - FacebookSDK@sdk-version-3.0.2
+    antifeatures:
+        Tracking:
+            en-US: Uses the Facebook SDK.
 
   - versionName: 2.1.1-c
     versionCode: 50