Make question marks optional

Fixes #96

android/uhabits-core/src/main/java/org/isoron/uhabits/core/preferences/Preferences.java

@@ -367,6 +367,10 @@ public class Preferences
         storage.putBoolean("pref_skip_enabled", value);
     }
 
+    public boolean areQuestionMarksEnabled()
+    {
+        return storage.getBoolean("pref_unknown_enabled", false);
+    }
 
     /**
      * @return An integer representing the first day of the week. Sunday