Support for reading Tags from protobuf definition files

[sophokles73]

The standard Tag importer supports reading tags from comments in a wide array of source code files.
Sadly, it is currently not possible to read Tags from protobuf definition files, which are used to define service interfaces and corresponding message types. Within these files, comments are used to describe the semantics of operations and request and response messages. It would be great if we could also put specification items in there which could be picked up by OpenFastTrace.

Comments in protobuf files are marked up using //. It should therefore be very easy to add support for protobuf definition files because all that would need to be done is add proto to the standard Tag importer's list of supported file extensions.

[sophokles73]

I could provide a corresponding PR if this would be a welcome addition.