First day of week

[FinnWinchester]

Add an option to configure which is the first day of week (currently Sunday by default, but I need it to be Monday).

[sunmix07]

Hello.
Calendar.js
var days = [].concat((0, _range2.default)(d1 - d2 + 2, d1 + 1), (0, _range2.default)(1, d3 + 1), (0, _range2.default)(1, 42 - d3 - d2 + 2));
var weeks = ['Пн', 'Вт', 'Ср', 'Чт', 'Пт', 'Сб', 'Вс',];