Provide a way to give extra data on announce

[yciabaud]

This PR adds a callback named getAnnounceOpts to the client options to be able to send updated data to the tracker (downloaded, uploaded, left and extra...).

The extra data can be handled in a tracker by listening to the event params.

I will send PRs for torrent-discovery and webtorrent in order to make this available in webtorrent.

[feross]

Nice! Thanks for coming with a pretty clean way to finally fix #3 (which has been open for almost 2 years!!)

My only concern is forcing the use of "extra" for the name of the http/websocket tracker parameter. Is this part of a BEP or some standard? If not, I'd rather just let the user specify any parameter names that they want. Just add the params the user specifies to the params dictionary using the xtend package (already in package.json)

Also, this needs documentation in the readme.

[feross]

This isn't necessary. All tracker types already have a reference to the Client instance. So they should check for self.client._getAnnounceOpts directly.

[yciabaud]

You're right, I was not comfortable with the use of extra neither but I didn't think about extending params. I am changing this.

[yciabaud]

Actually the getAnnounceOpts function looks like :

getAnnounceOpts: function () {
  return {
    uploaded: self.uploaded,
    downloaded: self.downloaded,
    left: self.length - self.downloaded,
    extraAnnounceOpts: {
      myOwnKey: myOwnValue,
      myOtherKey: myOtherValue
    }
  }
}

Do you believe I should flatten this as we are planning to mix it in the request?

[feross]

@yciabaud Yes, let's make it flattened everywhere.

[feross]

I'm now working on getting this cleaned up and merged.

[yciabaud]

OK sorry for the delay, I did work on the documentation and on flattening all this but I did not manage to test it thoroughly.

Do you want me to push these improvements in another PR?

[feross]

I've already fixed it up to flatten. Will push the changes here soon. Thanks for offering!

[feross]

7.3.0.

[yciabaud]

Ok thats great, its close to what I wrote on my side. I can add a documentation in the readme if you need.

[feross]

Sure, take a look at what's already there. Feel free to add to it :)