Custom config

[7] pry(main)> t = Time.now;100_000.times{Money::Custom.exponent('usd')};Time.now - t
=> 0.054287
[8] pry(main)> t = Time.now;100_000.times{Money::Currency.new('usd').exponent};Time.now - t
=> 0.38837

this allows us to put our custom configs in one place too, on top of Money gem's.