Implement new emacs command M-x gofmt

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

misc/emacs/go-mode.el

@@ -470,3 +470,9 @@ Useful for development work."
   (go-mode))
 
 (provide 'go-mode)
+
+(defun gofmt ()
+  "Pipe the current buffer through the external tool `gofmt`."
+  
+  (interactive)
+  (shell-command-on-region 1 (+ (buffer-size) 1) "gofmt" t t shell-command-default-error-buffer))