Add label_margin attribute to edges, and default_edge_label_margin to the diagram

[tk0miya]

Make the position of the label of the edge adjustable via label_margin attribute and default_edge_label_margin attribute.

before:

{ A -> B [label = blah]; B -> A [label = blah] }

after:

{
  A -> B [label = blah];
  A -> B [label = margin4, label_margin = 4, noactivate];
  B -> A [label = blah];
  B -> A [label = margin4, label_margin = 4, noactivate];
}