I think that this SNR value represents the most important output of this
example. There's plenty of debugging info on display already, so let's
make this one more prominent.
I was thinking about moving the highlighting to elements' each __str__()
function, but that felt a bit like layering violation to me. I could
have also changed just the transponder's __str__(). In the end, I think
that repeating just the final GSNR at the link-end transponder makes
most sense here. This is aligned with what we talked about at
yesterday's (on 2019-09-18 -- note that this is a backport from #261)
demo for Microsoft, after all.
I have no idea which one of the existing pypi modules is best for these,
and given that we're using just two escape codes, I think it makes sense
not to bother with a more capable third-party module just for two magic
strings.
