Make MAINTAINERS optional

[jonbodner]

The MAINTAINERS file is a historical relic that was inherited from the original LGTM project. We should have a section of the .checksout file that lets you specify your users and groups. Also, we should consider auto-importing all groups and users defined for the repo/org without any MAINTAINERS specification.

[shusak]

Please see #60

[jonathana]

Good step 1: if no MAINTAINERS file, fake it that you have one with a few default statements in it. Those should be github-org repo-self (validate exact syntax. Pull users from repo, org, and teams defined on that repo). @jonbodner can you provide us with what the "default" MAINTAINERS file that we should synthetically inject would be?