XEP-0392: theme, ui: conditionally colorize user nicks

* add theme_hash_attrs()
* when printing a user message check PREF_COLOR_NICK to decide whether
  to colorize it or not

ideally we should hash the jid instead of the nick but this is already
a first step.

src/config/theme.c

@@ -670,6 +670,12 @@ theme_free_string(char *str)
     }
 }
 
+int
+theme_hash_attrs(const char *str)
+{
+    return COLOR_PAIR(color_pair_cache_hash_str(str));
+}
+
 int
 theme_attrs(theme_item_t attrs)
 {