CONTRIBUTING.md generator

Replaces `slt-lint` with a multi-command `slt` that supports the `lint` and `cla` commands.

Generated CONTRIBUTING.md is based off of the one in strongloop/strongloop with modifications to replace commit signing with https://cla.strongloop.com/.

Will be used to make changes described in strongloop-internal/scrum-nodeops#53 to all our repos.