emacs: allow users to customize the gofmt command, in particular, to use goimports instead.
emacs: allow users to customize the gofmt command, in particular, to use goimports instead.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/23680043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang