numbers => [zero, one, two, three, four, five, six, seven, eight, nine,
    ten, eleven, twelve, thirteen, 15 => fifteen, 18 => eighteen,
    20 => twenty, 30 => thirty, 40 => forty, 50 => fifty, 80 => eighty
]
rules => [
    14,16,17,19 => {digit}teen,
    60,70,90 => {digit}ty,
    21-99 => {ten}-{digit},
    100,200,300,400,500,600,700,800,900 => {digit} hundred
]
exponents => [thousand, million, billion, trillion]
separator => ' '