Let user change log level while running

`/log level INFO|DEBUG|WARN|ERROR` is now available.
Looks like this solves a TODO (see removed comment in source) from 2013
:-)

Works only with default log file. Not with user provided log file during
start up via the -f parameter.

Fix https://github.com/profanity-im/profanity/issues/1627

src/command/cmd_ac.c

@@ -391,6 +391,7 @@ cmd_ac_init(void)
     autocomplete_add(log_ac, "rotate");
     autocomplete_add(log_ac, "shared");
     autocomplete_add(log_ac, "where");
+    autocomplete_add(log_ac, "level");
 
     autoaway_ac = autocomplete_new();
     autocomplete_add(autoaway_ac, "mode");