Add option to color MUC history like regular messages

`/logging group color` has:
* `unanimous` which will color it with one unanimous color. Like it was
done always.
* `regular` which colors it like regular incoming messages.

Regards https://github.com/profanity-im/profanity/issues/1261

src/config/preferences.h

@@ -165,6 +165,7 @@ typedef enum {
     PREF_OMEMO_SENDFILE,
     PREF_OCCUPANTS_WRAP,
     PREF_CORRECTION_ALLOW,
+    PREF_HISTORY_COLOR_MUC,
 } preference_t;
 
 typedef struct prof_alias_t {