Add Support for Custom Rulesets

[amthorn]

Custom rulesets is a standard with many linters and provides flexibility for users to create their own linting rules that may apply only to their company or project.

dockerfile lint -r ruleset.js Dockerfile

This would give me the flexibility to provide my own rulesets, which i am currently having to fork and fix for a project i'm working on. I submitted a PR for this here: https://github.com/replicatedhq/dockerfilelint/pull/172

This documents the underlying issue.