Feature Request: Add ability to not expand fields in the output

[markallanson]

We have hundreds of messages that contain a common "header" message.

The header has enough content that it clutters diagrams making it harder to view primary structure of the message being visualized.

It would be nice if there were a way to flag a field as being ignored for the purposes of visualization. When such a field is found, protodot would show the presence of the field in the parent message, but not generate a visualization tree of said message.