godoc: allow form feed in text files
godoc: allow form feed in text files

Now recognizes misc/emacs/go-mode-load.el as text.

Fixes issue 2115.

R=gri
CC=golang-dev
http://codereview.appspot.com/4801061
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go