Support DOT entry

[glebec]

The README example could be expressed in DOT format:

digraph G {
  a -> c
  a -> f
  b -> d
  b -> e
  c -> f
  d -> f
  d -> g
  e -> h
  f -> i
  g -> j
  h -> j
}

Allowing batching-toposort to accept DOT strings might be more convenient for some.