go/protobuf api v2: Should skycfg migrate to v2?

[dilyevsky]

Some pros/cons of the migration I could think of:

Pros:

• Cleaner/smaller codebase
• Dynamic message support (in relation to #79 (comment))

Cons:

• Unclear Gogo Protobuf support - duplicate both apis or drop support entirely?

[jmillikin-stripe]

See #15

If gogo doesn't support the standard Go protobuf library then we can work that out separately.