Add a deprecation flag for rules

[rgeyer]

It should be possible to mark an existing rule as "deprecated".

This would be dependent upon the dashboard-linter being properly versioned. See #150

Replacing/Removal Reason

When deprecated, a rule would optionally reference one or more rules which replace it. If no rules replace it, a description of why the rule is being removed should be provided.

Failure on deprecated rule

When a deprecated rule fails while linting a dashboard, the deprecation message should be included. Perhaps the rule should also be decreased in severity?

.lint/Configuration tidiness

When a deprecated rule is found in the config file (.lint, likely under the "exclusions" section), a message should be presented to the user that they're excluding a deprecated rule, and they may be able to remove the exclusion.

Perhaps we also need a "disabled" flag, which would allow still searching for the rule in the config (as above), but not execute the rule.