Feature: add marked dates

[hellpirat]

Add feature for adding marked dates.

Usage example:

markedDates={{
   '2018-12-03': { dots: [ { key: 1, color: 'red', selectedDotColor: 'blue' }, { key: 2, color: 'blue', selectedDotColor: 'red' } ] },
   '2018-12-04': { dots: [ { key: 3, color: 'green', selectedDotColor: 'blue' }, { key: 4, color: 'blue', selectedDotColor: 'red' } ] },
   '2018-12-05': { dots: [ { key: 5, color: 'white', selectedDotColor: 'blue' }, { key: 6, color: 'blue', selectedDotColor: 'red' } ] },
 }}

[peacechen]

Thanks @hellpirat for this handy feature. Some suggestions:

1. Add a screenshot. You can place one in this thread, then use the image link in the documentation.
2. For consistency with the rest of the library, markedDates should support any Moment paseable date value (string, Date or Moment object).
3. Change markedDates to an array of objects. Each entry would contain a date key along with the dots key, enabling support for #2 above.
4. Update getDateMarking to search the array for a matching date. Parse and compare the date with Moment (e.g. moment(date).isSame(day, 'day')
5. Set the default prop value for markedDates to an empty array. This reduces the error-checking code needed.

[hellpirat]

@peacechen Hi, I updated a code :) Please check it.

Usage example:

markedDates={[
  {
    date: '2018-12-26',
    dots: [
      { key: 1, color: 'red', selectedDotColor: 'blue' },
      { key: 2, color: 'blue', selectedDotColor: 'red' },
    ],
  },
  {
    date: '2018-12-28',
    dots: [
      { key: 1, color: 'red', selectedDotColor: 'blue' },
      { key: 2, color: 'blue', selectedDotColor: 'red' },
    ],
  },
  {
    date: moment(),
    dots: [
      { key: 1, color: 'red', selectedDotColor: 'blue' },
      { key: 2, color: 'blue', selectedDotColor: 'red' },
    ],
  },
]}

[peacechen]

Thanks @hellpirat
This looks great. I'll review some more and merge.

Update: Would you post a screenshot sample in this thread?

[hellpirat]

@peacechen

[peacechen]

@hellpirat Commits merged. I'll publish on npm shortly.

The marked / unmarked dates don't center vertically. It looks like you may need to add a blank spacer for dates that are unmarked, but only if the markedDates prop is provided. Would you like me to wait for you to fix that, or go ahead and publish?

[hellpirat]

@peacechen #123 I fixed and created a new pull request because of the previous merge.