X: equivalent to %r in en_US

[kawanet]

README.md description is NOT correct:

X: equivalent to %D in en_US (based on locale)
x: equivalent to %T in en_US (based on locale)

strftime.js implementation:

en_US: {
    days: ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday' ],
    shortDays: ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'],
    months: ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'],
    shortMonths: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'],
    ordinalSuffixes: [
        'st', 'nd', 'rd', 'th', 'th', 'th', 'th', 'th', 'th', 'th',
        'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th', 'th',
        'st', 'nd', 'rd', 'th', 'th', 'th', 'th', 'th', 'th', 'th',
        'st'
    ],
    AM: 'AM',
    PM: 'PM',
    am: 'am',
    pm: 'pm',
    formats: {
        c: '%a %d %b %Y %X %Z',
        D: '%m/%d/%y',
        F: '%Y-%m-%d',
        R: '%H:%M',
        r: '%I:%M:%S %p',
        T: '%H:%M:%S',
        v: '%e-%b-%Y',
        X: '%r',
        x: '%D'
    }
},