GraphvizOnline links should be produced

[EliahKagan]

This is effectively the same issue as EliahKagan/msf#4, but for this project (SubtreeSearch).

As now mentioned in the readme, GraphvizOnline (project) runs Graphviz in the user's browser, rendering DOT code as a graph drawing. This is very useful. Currently, SubtreeSearch includes complete DOT code in its output, but the user has to copy and paste it. Users may also be unaware of GraphvizOnline even if that would be the most useful available tool for their needs. So:

• The Crystal version could output a URL. Or, better, since the URL may be long, it should tell the user what command-line option to invoke it with to output a URL (and support that option).

  Most modern graphical terminals provide an easy way to open links (e.g., Ctrl+click). But it might be nice to also have a separate command-line option to cause it to open the GraphvizOnline URL in the default browser. If the default behavior ends up being to output a URL, then there should be an option for suppressing it.

• The C# version, being a LINQPad query, can and should always dump a GraphvizOnline hyperlink, which can be clicked. This is similar--conceptually, and from a user experience perspective--to codegraph's use of GraphvizOnline links. This is unobtrusive, yet also hard to miss and easy to click.