Add callback option

I have a stream that reads filenames from a directory, and in the `data` callback I load the files and do some processing on them. The stream however emits `end` as soon as the file list has been read, which is long before I've actually processed the input and called all the `.ticks()` I need.

I therefore needed a callback that I could be sure was run when the processing of all the files was actually done. This seemed like a nice solution, not making me have to implement counters and stuff when the progress bar already has 'em :)