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/config/preferences.h

@@ -176,6 +176,7 @@ typedef enum {
     PREF_URL_OPEN_CMD,
     PREF_URL_SAVE_CMD,
     PREF_COMPOSE_EDITOR,
+    PREF_SILENCE_NON_ROSTER,
 } preference_t;
 
 typedef struct prof_alias_t