editor: make editor configurable via /executable

`/executable set editor /full/path/to/edit`.

Regards https://github.com/profanity-im/profanity/issues/1521

src/command/cmd_ac.c

@@ -1044,6 +1044,7 @@ cmd_ac_init(void)
     autocomplete_add(executable_ac, "avatar");
     autocomplete_add(executable_ac, "urlopen");
     autocomplete_add(executable_ac, "urlsave");
+    autocomplete_add(executable_ac, "editor");
 }
 
 void