function getFormatter(formatObj, separators) {
    formatObj.decimal = separators.charAt(0);
    formatObj.thousands = separators.charAt(1);

    return {
        numberFormat: function(formatStr) {
            try {
                formatStr = formatLocale(formatObj).format(
                    Lib.adjustFormat(formatStr)
                );
            } catch(e) {
                Lib.warnBadFormat(formatStr);
                return Lib.noFormat;
            }

            return formatStr;
        },
        timeFormat: timeFormatLocale(formatObj).utcFormat
    };
}
getFormatter: combine the final separators with the locale formatting object
we pulled earlier to generate number and time formatters
TODO: remove separators in v3, only use locale, so we don't need this step?

@param {object} formatObj: d3.locale format object
@param {string} separators: length-2 string to override decimal and thousands
  separators in number formatting

@returns {object} {numberFormat, timeFormat} d3 formatter factory functions
  for numbers and time