Streamed signing and archiving

[thom4parisot]

• crypto.sign apparently accepts ReadableStream
• generatePackage could return a stream instead of a Buffer object

The entire archive must have been fully read to generate a valid signature, and read again (?) to emit a signed stream.

new Crx(…).load('./dir').then(function(crx){
  crx.generatePackage().pipe(fs.createWriteStream('./dist/package.crx'));
});

[ahwayakchih]

@oncletom maybe something like i did here: https://github.com/ahwayakchih/crx3/blob/master/lib/crx3stream.js

In short: implement WriteStream, that simply updates sign and passes data to file (after leaving enough space for a header data), but at the end it finishes creating CRX header and writes it at the beginning of file.