Add multiple classes support for "CSSClasses" parameter.

[olegpix]

Closes #247

Hi, Thanks for the great plugin. I've added some improvements so that you can use the CSSClasses property with multiple classes. This is very useful for the headless ui approach, such as using a Tailwind CSS.

• Ability to add multiple classes to "dayBtn" using the "CSSClasses" parameters.
• Ability to add multiple classes to "day" using the "CSSClasses" parameters.
• Ability to add multiple classes to "arrow", "arrowPrev", "arrowNext" using the "CSSClasses" parameters.
• Ability to add multiple classes to "week", "weekDay", "weekDayWeekend" using the "CSSClasses" parameters.
• Ability to add custom icon to the "arrowPrev" and "arrowNext" via "iconsSet" parameter.

PS: Please uncomment the code inside the demo/main.ts file to test the improvements and the new added iconsSet parameter.

[uvarov-frontend]

Thanks for your input, I see you have added partial support for multiple css classes. I think we need to finish the work and transfer everything to data attributes.

[uvarov-frontend]

There are too many conflicts with the new code, I'll create a new branch from the current version, thanks for your input.