[Kotlin] Calendar extensions

간단

val Calendar.year: Int
    get() = this.get(Calendar.YEAR)

val Calendar.month: Int
    get() = this.get(Calendar.MONTH)

val Calendar.day: Int
    get() = this.get(Calendar.DAY_OF_MONTH)

val Calendar.dayOfWeek: Int
    get() = this.get(Calendar.DAY_OF_WEEK)