[ safety ] Treat extra args in configs a bit smarter

[buzden]

See discussion in #270. We should treat single strings of extra configs as being able to pass several arguments, while disallowing unsafe things like leaving unescaped & and similar things. Simple stuff like splitting by a space can brake options containing spaces inside strings like "--opt1 --opt2=\"a b\"".

As an author of status quo, I'm going to think how can we sort this out in December, if no one would come with a solution earlier.