Add option to only allow messages from jids in roster

`/silence on` will throw away all messages (type: chat, normal) that
come from jids that are not in the roster.

Implement https://github.com/profanity-im/profanity/issues/955

src/command/cmd_funcs.c

@@ -9505,3 +9505,11 @@ cmd_editor(ProfWin* window, const char* const command, gchar** args)
     }
     return TRUE;
 }
+
+gboolean
+cmd_silence(ProfWin* window, const char* const command, gchar** args)
+{
+    _cmd_set_boolean_preference(args[0], command, "Block all messages from JIDs that are not in the roster", PREF_SILENCE_NON_ROSTER);
+
+    return TRUE;
+}