Add Support for warn and deny Lint Configuration

[InsertCreativityHere]

Currently we support 'allowing' lints (which downgrades them to a diagnostic level of Allowed).
This can be done with either the [allow(X)] attribute or the --allow X command line option.

To be feature complete we should also add support for warn and deny attributes/CLI options.

Like allow, these would only work for lints (not errors):
Marking a lint as warn will cause the compiler to emit warnings for the lint.
Marking a lint as deny will cause the compiler to emit errors for the lint.

Note that unlike 'true errors', just because a lint is marked with deny, it will not cause compilation to terminate early.
These lint configuration attributes only affect the compiler's exit code, and the message we emit for the lint.
It has no other affect on compilation.

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An open question is what do we use for the short-command-line-options?
-a == --allow, -w == --warn, -d == --deny?
or
-A == --allow, -W == --warn, -D == --deny?

Rust users upper-case for these, but we're already using -D for --define.

An alternative is instead of using deny we could name it error, giving us --error and -E which are free options.