Support chunked file writing

[adam-azarchs]

This is really a superset of #4.

Currently,

ar/writer.go

Lines 93 to 96 in 809d437

	if len(b)%2 == 1 { // data size must be aligned to an even byte
	n2, _ := aw.w.Write([]byte{'\n'})
	return n+n2, err
	}

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.