add new boolean GH_COMMENT env var for whether to post GitHub comments

[snarfed]

this is useful because we just use Shamer as a leaderboard that aggregates coverage contributions by user. we use Coveralls to collect and track coverage over time, which is great, so we don't need Shamer for that. setting this new env var to false lets us skip the GitHub comments, since Coveralls is already a status check on our GitHub PRs, and also lets us avoid storing coverage data in AWS at all.

[snarfed]

thanks again for merging the first PR. friendly ping on the rest!

[yasyf]

@snarfed thanks again for the contributions! left comments on all 3 remaining PRs.