Extend Go styleguide

I've added some more sections to this. This is mostly based on my experience writing Go (not in GDS), so the usual disclaimers about aspects only being locally relevant apply :)

I was hoping @jabley could comment on logging and @mattbostock on testing (I haven't used ginkgo etc.)

Not sure how useful the section on frameworks is. Thoughts?

Although there are TODOs, this might be useful enough to merge?