Represents any number of values formatted using the first argument,
separated by the second format argumewnt in serialized form. Similar
to the usual sepBy combinator.
>>> testParse (takeCharsWhile isLetter `sepBy` literal ",") "foo,bar,baz"
Right [([],"foo,bar,baz"),(["foo"],",bar,baz"),(["foo","bar"],",baz"),(["foo","bar","baz"],"")]