Check brick measurements and findings against thresholds.

The config maps each report section (:dependencies, :complexity,
:modularity, :io, :errors, :tests) to its metrics, and each metric to
its settings: a :warning and an :error threshold, and any options the
metric has, such as :allow for :library-spread. A threshold is a
number, or for a metric of a brick's own value, {:std-devs k}, which
flags a brick more than k sample standard deviations past the mean of
the other bricks the metric checks (with at least :min-peers of them,
default 3).

What a threshold means comes from the metric (see metrics/metrics): its
:direction says whether values above (:max) or below (:min) the limit
are flagged, its :checks which brick types are checked, and its :kind
what is compared. A value past both thresholds is an error. A nil
threshold turns that level off, and a nil metric turns the metric off.