Create a CLI to convert a pipeline to graphviz dot format

As a Beam developer (especially one using python or go) I want a way to convert a pipeline to graphviz dot format so that I can easily visualize it.  

The Beam source comes with a great utility in the form of PortablePipelineDotRenderer.java. It seems like a great util to wrap with a CLI, so that it can be easily accessed from other languages and just for general ease-of-use for java developers.

The tool would take an input pipeline message as either a path to a file or via stdin, and would output a dot string to stdout or to a file.

I'm happy to make a PR for this.