Add /occupants color command

`/occupants color on|off` to enable or disable XEP-0392 also for the
MUC occupants.

Regards
https://github.com/profanity-im/profanity/issues/1191

src/config/preferences.c

@@ -1712,6 +1712,7 @@ _get_group(preference_t pref)
         case PREF_CONSOLE_CHAT:
         case PREF_COLOR_NICK:
         case PREF_ROSTER_COLOR_NICK:
+        case PREF_OCCUPANTS_COLOR_NICK:
         case PREF_STATUSBAR_SHOW_NAME:
         case PREF_STATUSBAR_SHOW_NUMBER:
         case PREF_STATUSBAR_SELF:
@@ -1975,6 +1976,8 @@ _get_key(preference_t pref)
             return "color.nick";
         case PREF_ROSTER_COLOR_NICK:
             return "color.roster.nick";
+        case PREF_OCCUPANTS_COLOR_NICK:
+            return "color.occupants.nick";
         case PREF_BOOKMARK_INVITE:
             return "bookmark.invite";
         case PREF_PLUGINS_SOURCEPATH:

src/config/preferences.h

@@ -145,6 +145,7 @@ typedef enum {
     PREF_CONSOLE_CHAT,
     PREF_COLOR_NICK,
     PREF_ROSTER_COLOR_NICK,
+    PREF_OCCUPANTS_COLOR_NICK,
     PREF_BOOKMARK_INVITE,
     PREF_PLUGINS_SOURCEPATH,
     PREF_ROOM_LIST_CACHE,