Doesn't throw errors on bad urls (just returns them without modification)
Will only apply changes on the domain/host you specify.
var createAdderFunction = require('./')
var addAffiliateCode = createAdderFunction('shoppingsite.com', 'code', 'my-money-making-code')
addAffiliateCode('http://shoppingsite.com/thing') // => 'http://shoppingsite.com/thing?code=my-money-making-code'
addAffiliateCode('http://google.com/thing') // => 'http://google.com/thing'