Add option for onFirst

[JacobDB]

I'm working on a gulp task that uploads files to a remote server using vinly-ftp. I'd like to notify the dev using this task of where files are being uploaded to before the upload starts, so that they can quickly ctrl+c out of it if they realize the destination is incorrect (or at least know where to go to fix anything that broke).

I have this mostly working, except it fires on every single file in the stream, which is definitely overkill. I'd much rather have it trigger for only the first file; would it be possible to add an option onFirst similar to the existing onLast option?