go.tools: Expose filename rather than extension only, needed for new tour.

R=adg, r
CC=golang-codereviews
https://golang.org/cl/43470050

present/code.go

@@ -31,6 +31,7 @@ func init() {
 type Code struct {
 	Text     template.HTML
 	Play     bool   // runnable code
+	FileName string // file name
 	Ext      string // file extension
 	Raw      []byte // content of the file
 }
@@ -130,6 +131,7 @@ func parseCode(ctx *Context, sourceFile string, sourceLine int, cmd string) (Ele
 	return Code{
 		Text:     template.HTML(buf.String()),
 		Play:     play,
+		FileName: filepath.Base(filename),
 		Ext:      filepath.Ext(filename),
 		Raw:      textBytes,
 	}, nil