Export copy function

Per https://github.com/mapbox/tilelive.js/issues/95

cc @rclark 
The tilelive.WriteStream is a funky stream. Usually a node writable stream fires a `finish` event when it is done writing everything, but the tilelive stream is an exception. It'll fire `finish` *when you've asked it to write every tile that you're going to ask it to write*, but it will later fire a `stop` event that means *it has actually finished writing all the tiles*

Short story is, this should be `.on('stop',...` and I knew this and I'm sorry I never fixed in in bin/tilelive-copy