hydiomatic.rules: Move the warning rules to a separate set

Since warning rules print stuff themselves, they cannot be intermixed
with other rules, so move them to a separate set. This partially
addresses #7.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
bin/hydiomatic.hy: Add a --warnings option

With the warnings moved to a separate rule set, we need a way to run our
code against them. The --warnings option does just that. It does not run
any other ruleset, just this.

This fixes #7, though not in the way described there, as the results are
not interleaved, but still.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>