Add option to hilight unread rooms in /wins command

In the theme we can now set `cmd.wins.unread` to hvae a special color
for the lines of the `/wins` output that have unread messages.

Fix https://github.com/profanity-im/profanity/issues/895

src/config/theme.h

@@ -137,6 +137,7 @@ typedef enum {
     THEME_MAGENTA,
     THEME_MAGENTA_BOLD,
     THEME_TEXT_HISTORY,
+    THEME_CMD_WINS_UNREAD,
 } theme_item_t;
 
 void theme_init(const char *const theme_name);