Add setting to not colorize own nick according to xep-0392

Some users might want there nick to always stay white (etc) for easier
recognition.

Now we can do `/color own off` to not generate the color based on
xep-0392. The `me=` color (etc) from the theme will then be used.

Once we run this command `theme_load()` is called again.
And the theme looks totally wrong.
We encountered this at other times already and I think it's nothing
wrong with this new code here now but that there seems to be a missing
closing attr for the color when drawing.

Should be investigated seperately.

Fix https://github.com/profanity-im/profanity/issues/1288

src/command/cmd_ac.c

@@ -958,6 +958,7 @@ cmd_ac_init(void)
     autocomplete_add(color_ac, "off");
     autocomplete_add(color_ac, "redgreen");
     autocomplete_add(color_ac, "blue");
+    autocomplete_add(color_ac, "own");
 
     correction_ac = autocomplete_new();
     autocomplete_add(correction_ac, "on");
@@ -3758,6 +3759,11 @@ _color_autocomplete(ProfWin *window, const char *const input, gboolean previous)
 {
     char *result = NULL;
 
+    result = autocomplete_param_with_func(input, "/color own", prefs_autocomplete_boolean_choice, previous, NULL);
+    if (result) {
+        return result;
+    }
+
     result = autocomplete_param_with_ac(input, "/color", color_ac, TRUE, previous);
     if (result) {
         return result;