add DotHighlightedDigraph method

[flsmith]

This PR adds a method for producing a dot string where a subdigraph is highlighted.

I'm open to comments if people would like this to work differently - I wrote it this way for a specific application, and it might not be the best generic solution.

[james-d-mitchell]

This PR is very welcome. I wonder if it might not be a more consistent approach to simply allow a colouring of the vertices of a digraph be the second argument to DotDigraph? This would be quite natural, and also serve your purpose, example usage:

DotDigraph(D, [1, 2, 3], ["blue", "red", "pink"]);

Since there is built in functonality for checking if a coloring is valid, this might be one way to go. What do you think @wilfwilson and @flsmith ?

[wilfwilson]

I like what you've done @flsmith

@james-d-mitchell I think your suggestion serves a different purpose. @flsmith's code has nothing to do with graph colouring, it's about emphasising a particular induced subdigraph in a digraph: the special vertices and the edges between them get a special colour. To bring graph colouring into it is something quite different, and I don't see how it is compatible with what @flsmith has done.

[flsmith]

I've discussed this with @james-d-mitchell and he's happy with this as it is.

I'll create a new issue for giving arbitrary colours to vertices/edges, which might be useful too.