Added specialClass styling and implementation

[amattu2]

For my personal usage, I needed a way to indicate holidays in a minor way, as such:

Implemented a specialClass styling and JavaScript implementation. By default, it appears as a 5px by 5px dot under the date. This could be used to symbolize a special date or otherwise.

See December 24th, 2019 in the below screenshot for default styling

From the CHANGELOG.md

# Changelog

## 1.1.0 (Dec 26, 2019)

- Added `special` function and associated CSS

### Methods

- Added 1 new method: `special`

From the documentation:

### specialClass

- Type: `String`
- Default: `special`

A class (CSS) for the specialClass date item.


### special

- Type: `Function`
- Default: `null`
- Syntax: `special(date, view)`
  - `date`: the date for checking.
  - `view`: the the current view, one of `day`, `month` or `year`.

Add `specialClass` class to date item. If return a `true` value, the `specialClass` class will be added to the item. The default style is a 5px by 5px black dot under the date text item.

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