Support chunked file writing

[adam-azarchs]

This is really a superset of #4.

Currently, https://github.com/blakesmith/ar/blob/809d4375e1fb5bb262c159fc3ec2e7a86a8bfd28/writer.go#L93-L96 essentially closes the file after each call to Write, which means you can't use the API to write a file one piece at a time (e.g. with io.Copy, or dumping out from a stream),

It should only write that footer when aw.nb == 0.

In my opinion, the best way to handle this would be to follow the example of archive/tar and don't write the file footer until either the next call to WriteHeader() or Close(). Of course that would require introduction of a Close method, which would be a breaking API change.