Add ability to track progress of packing

[coderaiser]

Would be great to have ability of tracking progress of packing. When we have a lot files, operation could keep going really long time. For example tar-fs has optional map function which is called on every entry.

Actually yazl is EventEmitter from the inside, so it should not be a problem emit every processed entry.

Yauzl has it by the way, very useful feature.