PR approvals with LGTM

[levithomason]

There are a few contributors now and a likely increase in activity coming to this project. To help wrangle PR approvals, I propose we add LGTM.

We add a simple .lgtm in the root that lists contributors. Then, PR statuses will then require approval from contributors by adding a comment with "LGTM" in it. The number of required approvals is configurable. Also, if commits are added after approvals were given, status is reset.

You can see it used at shelljs and enclave for examples.

If we agree this is a good move, I'll set it up right away.

[levithomason]

With action being a little slow still, opting not add more process or blockers to progress for now.