Add Method of Using External Rulesets

On several projects that I've worked on recently we've had a need to use code sniffer sets that are not included in the coding-standards that power the Linter Bot. These have included Security sniffs, VIP-specific sniffs, and more.

As a developer, we need to have a way to reference custom rules created for a project and CS sets that are not present in the bundle deployed to the Linter Bot.