misc/emacs: allow users to customize path of 'go' tool.
misc/emacs: allow users to customize path of 'go' tool.

Some users have multiple Go development trees and invoke the
'go' tool via a wrapper that sets GOROOT and GOPATH based on
the current directory.  Such users should customize go-command
to point to the wrapper script.

R=dominik.honnef
CC=golang-dev
https://codereview.appspot.com/13233043

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