Add support for multi-line strings

[maguro]

I am shepherding a change in Protobufs to allow multi-line string values:

https://github.com/protocolbuffers/protobuf/issues/1297

This ticket serves as a marker for the corresponding effort required in this project. I have forked this project and gotten a working version on my fork. I'll submit a PR once things start solidifying in the protobuf project.

[kshchepanovskyi]

Wow, you did great work at making protobuf support multiline strings at https://github.com/protocolbuffers/protobuf/issues/1297!

I started work on adding support for "multipart" strings couple of weeks ago, but it appeared to be more challenging that I thought (string like option x = "this" "is" "a" "string";). Somehow protoc supports this format everywhere (however, I might be mistaken).

[cosmotek]

Any updates here?