Define a new interactive function in go-mode.el called gofmt.
This function passes the current buffer through the external `gofmt`
tool and replaces the buffer with the result.
R=agl, rsc
https://golang.org/cl/154044
for keywords, built-ins, functions, and some types. It also
provides indentation that is (almost) identical to gofmt.
R=rsc
APPROVED=rsc
DELTA=509 (509 added, 0 deleted, 0 changed)
OCL=35951
CL=35994
