Add multiple classes support for "CSSClasses" parameter.

[olegpix]

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.