Optionally switch to proto text format parser for conformance tests

I ended up getting distracted today and wrote a textproto parser in Ruby using Racc. It probably has bugs in it, but it's able to parse all the textproto files aside from proto2_ext.textproto, and I'm not seeing any conformance test failures that seem to be caused by parsing bugs in my rough review.

Feel free to use it if you want to. I ended up writing it because it simplifies the need to use Python to update the testdata files, and eventually might enable proto2 extension support (although the Ruby protobuf gem has some limitations that might make that impossible).