Close sink.GetWriter results in zipextractor

[200sc]

Fixes #4

[fasterthanlime]

The way the savior API is currently designed, extractors should be used like so:

func something() {
  sink := makeSink()
  defer sink.Close()
  
  res, err := getExtractor().Resume(nil, sink)
  // handle err etc.
}

One could argue otherwise, but I think this follows a pattern often found in Go libraries: if you open a file, you're in charge of closing it - whatever you pass it to shouldn't close it for you.

Extractors should call entryWriter.Sync() when they checkpoint, to make sure the checkpoint actually corresponds to what's on disk, but they don't have the responsibility to close any writer or sink.

See: https://github.com/itchio/butler/blob/b38feca3f276c0c9aa15de94a321cbc9bdab401f/archive/simple_extract.go#L16-L48