cmd/trace: set cname for span slices

Define a set of color names available in trace viewer

https://user-images.githubusercontent.com/4999471/37063995-5d0bad48-2169-11e8-92be-9cb363e21c38.png

Change-Id: I312fcbc5430d7512b4c39ddc79a769259bad8c22
Reviewed-on: https://go-review.googlesource.com/99055


Reviewed-by: Heschi Kreinick <heschi@google.com>