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

webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,scirelli/scirelli-go,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,scirelli/scirelli-go,d0f/go-zh,d0f/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,mhennings/marcohennings-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh