function mapDomain(string, fn) {
		return map(string.split(regexSeparators), fn).join('.');
	}
A simple `Array#map`-like wrapper to work with domain name strings.
@private
@param {String} domain The domain name.
@param {Function} callback The function that gets called for every
character.
@returns {Array} A new string of characters returned by the callback
function.