Add a linter to require RFC-2119 keywords are capitalized

[kdenhartog]

This isn't a necessity, but it would be nice if the linter called out when keywords are used and forced capitalization of them. This can be useful for readers to understand when the reader intentionally wants a piece of text to be a normative statement (required for conformance to the spec) versus when the author is accidentally reusing a normative word and should update the text so that it's more clear this isn't required.

[SamWilsn]

So the two rules could be:

• Error if capitalized keywords appear outside of the specification section.
• Warn if a non-capitalized keyword appears inside the specification section.

Sound reasonable?

[kdenhartog]

That would be ideal. I'd be alright with warn for both as well based on the amount of complaints about linter being overly strict.