Add a whitelist option for `require-description`

[brettz9]

While there is currently an exemptedBy blacklist for require-description to disable the rule when certain tags are present on a block, it would be nice to be able to disable this rule by default while still having it apply when specific desired whitelisted tags are present, e.g., @typedef.

The other rules allowing exemptedBy (require-example, require-param, and require-returns) may also benefit from such a whitelist (and should, as with exemptedBy, be easy enough to apply to all without extra work).

However, it would have to be done in a context-aware way, I think, e.g., requiring the presence of typedef in an anywhere context to require a description, but not requiring any tag to expect a description on say a function declaration context. (Perhaps someone would also like exemptedBy made context aware as well.)

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