Format number with metric prefix.

format_metric(14     , {base=>10});               # => "14"
format_metric(12000  , {base=> 2, precision=>1}); # => "11.7K"
format_metric(12000  , {base=>10, precision=>1}); # => "11.7Ki"
format_metric(-0.0017, {base=>10});               # => "1.7m"