add api to allow upload progress monitoring

[acornejo]

This is fairly straightforward to do with superagent (which is used by this sdk).

request
   .post('/files/upload')
   .send(file)
   .on('progress', function(e) {
      console.log(e.percent);
   }

Thus just need to change the API to allow the user to specify a progress callback.

[greg-db]

Thanks! I'll send this request along to the team.

[nikshepsvn]

Are open source contributions for these type of issues allowed @greg-db ?

[greg-db]

@nikshepsvn Sure, we welcome pull requests.

[pran1990]

@nikshepsvn thanks for offering, if you were. One thing I'd like to warn you about is that we are moving to fetch soon, so a change dependent on superagent might be really short-term for you.