Ability to downgrade error level to "warning" instead of just "error"

[CNovakPB]

Some rules are important to be made aware of, but a team of developers may not have the bandwidth to resolve all of them in a repo at one time. Therefore, it would be helpful to be able to set a rule's level to "warning" instead of just "error." This mimics the abilities of eslint and stylelint.

With a "warning," a test suite written in ember-cli and qunit can still pass without failure. With only "errors" in ember-cli-template-lint, the test suite will fail, and prevent the codebase from moving forward (particularly in a CI system). At that point, the developer is forced to disable the template rule completely, and new instances of the rule being broken can/will be added to the codebase, making the eventual refactor much worse.

Please add the ability to individually downgrade errors to warnings, in addition to completely disabling a rule.