v5.5.0: Add support for paths=source_relative option

This release adds support for a new generator option, paths=source_relative, to match the behavior of the official protoc-gen-go generator. That behavior is documented at golang/protobuf#533.

Now, protoc-gen-twirp can work outside of GOPATH to generate code. Files are output relative to where the proto source was found.

The change to make this possible is #126.