Parse delimited from test

Hi, I need to parse multiple messages from one file. In Google protocol buffers I used [parseDelimetedFrom](https://developers.google.com/protocol-buffers/docs/reference/java/com/google/protobuf/AbstractParser#parseDelimitedFrom(java.io.InputStream)). This is my attempt to implement same method wire-way.
I would take a `Sink` as the argument and call `Okio.buffer` to get a `BufferedSink`. The next version of Okio won't double-buffer and I value the cleanliness of the API over the additional buffering we incur currently.