Allow dumping varint-delimited protos

[timothyg-stripe]

Currently, protoscope doesn't allow dumping or representing varint-delimited protobuf messages, which is used somewhat frequently in parts of the world. E.g., some official protobuf libraries has parseDelimitedFrom and writeDelimitedTo; Bazel uses it for query --output=streamed_proto and build --build_event_binary_file=.... It would be helpful to add support for these dumps, behind an option of course.

A simple extension to the protoscope language to allow representing varint-delimited protos could be the --- document separator used by YAML.