Feature Request: Output any env vars that are picked up

[jabrown85]

There are lots of GO<var> options available to users of go. Some of these are honored by the Heroku go buildpack and some may not be at this time. An example I'm unsure about it is GOEXPERIMENT

I can set GOEXPERIMENT=loopvar on my application - but there isn't a way to tell in the buildlog if that was picked up or not. I propose that when consuming config vars, the buildpack output the key name so we can be confident it built with the value set or used by the buildpack.

Something like this...

Found GOEXPERIMENT, GOVERSION