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/config/preferences.h

@@ -151,6 +151,7 @@ typedef enum {
     PREF_CONSOLE_PRIVATE,
     PREF_CONSOLE_CHAT,
     PREF_COLOR_NICK,
+    PREF_COLOR_NICK_OWN,
     PREF_ROSTER_COLOR_NICK,
     PREF_OCCUPANTS_COLOR_NICK,
     PREF_BOOKMARK_INVITE,