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://codereview.appspot.com/7085054