Export copy function

Per https://github.com/mapbox/tilelive.js/issues/95

cc @rclark 
The reporting in `bin/tilelive-copy` depends on metrics from `get.stats` and the details returned by the progress-stream (`p`). Why don't we bundle these up into an object so that in barely psuedo code:
```
var stats = get.stats + p;
opts.progress(stats);
```