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

pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go