Enhance handler customization.

[ldez]

I would propose to you this PR fixing #43, #40, #34:

• added 2 new methods: NewGzipHandler, NewHandler
  • with functional options
  • deprecated old method (I can change if you want)
• added a GzipWriter interface
  • allow some customization (like asked in #34, #40)

This changes:

• don't expose previous unexported methods or struct. (like acceptsGzip or others)
• don't break current behaviors.

example

```golang package gziphandler import ( "bufio" "log" "net" "net/http" ) type GzipResponseWriterLogger struct { GzipResponseWriter } func (g *GzipResponseWriterLogger) Write(b []byte) (int, error) { log.Printf("Write: %v\n", b) log.Printf("ResponseWriter Header: %v\n", g.ResponseWriter.Header()) return g.GzipResponseWriter.Write(b) } func (g *GzipResponseWriterLogger) WriteHeader(code int) { log.Printf("WriteHeader: %v\n", code) g.GzipResponseWriter.WriteHeader(code) } func (g *GzipResponseWriterLogger) Hijack() (net.Conn, *bufio.ReadWriter, error) { log.Println("Hijack") return g.GzipResponseWriter.Hijack() } func (g *GzipResponseWriterLogger) Close() error { log.Println("Close") return g.GzipResponseWriter.Close() } func (g *GzipResponseWriterLogger) Header() http.Header { log.Println("Header") return g.GzipResponseWriter.Header() } func GzipHandlerLogger(h http.Handler) http.Handler { wrapper, _ := NewHandler(&GzipResponseWriterLogger{}, CompressionLevel(2), MinSize(1024)) return wrapper(h) } ```

Fixes #43, #40, #34

[jprobinson]

Hi there, @ldez! Sorry for the delay, I was at GopherCon last week.

While we're interested in using functional options for this package, I'm not quite sure the addition of this interface is really needed. Do you think you could implement this without it?

[jprobinson]

Sorry, meant to just comment, not close!

[ldez]

The interface GzipWriter (the name is not the best but GzipResponseWriter is already used) allow to add some custom behaviors without needing to add them to this middleware.

I'm not really happy what I did with this interface (unexported methods) but I don't see a cleaner way to do that, if you have an idea I can implement.

But if you think it's not a interesting idea, I can drop this part.

WDYT?

[ldez]

With the merge of #51, functional options have been added. And without any response I suppose you don't want my PR.