Modifiers to match day of the week

[gpbl]

We can extend the type of modifiers to accept days of the weeks.

A common case is to match all the Sundays. This can be done using a function:

const sunday = day => day.getDate() === 0 
<DayPicker disabledDays={ sunday } />

Instead, we should be able to write:

// Disable Sundays
<DayPicker disabledDays={{ daysOfWeek: 0 }} />

// Disable Sundays and Saturdays
<DayPicker disabledDays={{ daysOfWeek: [0, 6] }} />

• implement new modifier type
• add unit tests
• update docs
• update examples
• update typescript definitions file