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.c

@@ -1916,6 +1916,7 @@ _get_group(preference_t pref)
     case PREF_TLS_CERTPATH:
     case PREF_CORRECTION_ALLOW:
     case PREF_MAM:
+    case PREF_SILENCE_NON_ROSTER:
         return PREF_GROUP_CONNECTION;
     case PREF_OTR_LOG:
     case PREF_OTR_POLICY:
@@ -2198,6 +2199,8 @@ _get_key(preference_t pref)
         return "url.save.cmd";
     case PREF_COMPOSE_EDITOR:
         return "compose.editor";
+    case PREF_SILENCE_NON_ROSTER:
+        return "silence.incoming.nonroster";
     default:
         return NULL;
     }

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