app: use factored import in default snippet
app: use factored import in default snippet

This is a slightly more convenient starting point when adopting this
snippet for your own needs.

Fixes golang/go#13813

Change-Id: I0ece3d811d77c82121a90f52823b69b35f4a6e2c
Reviewed-on: https://go-review.googlesource.com/18189
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

gophergala2016/playground-plus,gophergala2016/playground-plus,gophergala2016/playground-plus,golang/playground,golang/playground,golang/playground