cmd/godoc: support m=text parameter for text files

It's possible to view the package docs in plain text, eg:
        http://golang.org/pkg/time/?m=text
and this CL introduces the ability to do the same for files:
        http://golang.org/src/pkg/time/time.go?m=text

R=golang-dev, dave, minux.ma
CC=golang-dev
https://golang.org/cl/7085054

src/cmd/godoc/godoc.go

@@ -15,6 +15,7 @@ import (
 	"go/format"
 	"go/printer"
 	"go/token"
+	htmlpkg "html"
 	"io"
 	"io/ioutil"
 	"log"
@@ -681,10 +682,16 @@ func serveTextFile(w http.ResponseWriter, r *http.Request, abspath, relpath, tit
 		return
 	}
 
+	if r.FormValue("m") == "text" {
+		serveText(w, src)
+		return
+	}
+
 	var buf bytes.Buffer
 	buf.WriteString("<pre>")
 	FormatText(&buf, src, 1, pathpkg.Ext(abspath) == ".go", r.FormValue("h"), rangeSelection(r.FormValue("s")))
 	buf.WriteString("</pre>")
+	fmt.Fprintf(&buf, `<p><a href="/%s?m=text">View as plain text</a></p>`, htmlpkg.EscapeString(relpath))
 
 	servePage(w, Page{
 		Title:    title + " " + relpath,