Add a strongly typed javascript output

[moul]

The current JS output isn't strongly typed

[cerisier]

Feels like this is more of second pass rather than a first class protoc output. Using this https://github.com/ngbrown/protoc-gen-dts ?

While we're at it, let's do Flowtype as well (We are using Flowtype vs Typescript internally at Zen.ly) Using this https://github.com/flegall/protobuf-js-typer ?

There are also the following options:

• https://github.com/SINTEF-9012/Proto2TypeScript
• https://github.com/netproteus/protobuf2flowtype