Add protobuf support to @xviz/io

[twojtasz]

• Initial support for a file with a magic header PBE1 followed by our protobuf envelope and other types.
• Enumerations are not handled most efficiently so we are converting to a Javascript Object at a performance penalty in this change. Will address in a follow-up
• Primitive color is normalized to an array of bytes prior to encoding in Protobuf. This creates some roundtrip testing issues. The plan is to change color in a follow-up to bytes.
• We have a custom protobuf.js module that can decode arrays to TypedArrays, which will be addressed in a follow-up

[jlisee]

See comments on #499 since I think the same code is in both PRs (probably meant to stack these)

[coveralls]

Coverage increased (+1.9%) to 63.356% when pulling a1c1232 on tpw/add-protobuf-support into 0d1acce on master.