Allow left-associative operators not to go at the end of a line

[Niols]

As of f35eee4, Topiary formats:

foo
  bar
  baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz
  % int_of_string

and

foo
  bar
  baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz
  %
  int_of_string

as:

foo
  bar
  baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz %
  int_of_string

Note how the left-associative % operator goes at the end of the very long argument baaa[...]aaaz, making it much harder to spot and making it easier to mistake int_of_string for an argument of foo. I understand the rules about left- and right-associativity of operators making them go at the beginning/end of the lines, so maybe the very first snippet should not be allowed, but maybe we should allow operators standalone on their lines? Really not sure about this, so I marked this as feature request and not bug.