Generate ES6-style code (classes, exports, imports)

[gonzojive]

There is a long-standing request for ES6 module generation: https://github.com/protocolbuffers/protobuf-javascript/issues/75. This PR mostly adds such support, though it needs to be cleaned up, fixed and tested.

The code generator should still produce the same output for previously supported inputs. The es6 option changes the output style significantly.