scratch: Added sm command
scratch: Added sm command

This command implements step 5 of https://golang.org/s/gophercon2017

Change-Id: I9c973bff93eb5ce8d853bde584bf34a22ebba856
Reviewed-on: https://go-review.googlesource.com/48722
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/scratch