Allow global downgrade to warning

lint-trap now supports attenuating rules to warnings and only warning lint messages produce a 0 exit-code

this closes:
https://github.com/uber/lint-trap/issues/20
https://github.com/uber/lint-trap/issues/9

AFAICT this should now be good enough to use now in all web-core and realtime dev productivity projects. Please review this and I'll publish the first version after this is merged into master.

@Raynos @mlmorg