Custom Checkbox built

[jlnrrg]

I recently tried to built a tristate checkbox with customized Icons and colors.
Unfortunately this is impossible to achive with the Checkbox, thus I came back to this package and noticed a few possible improvements:

• bring the readme to the most recent version (some code examples are wrong)
• make the icon in AnimatedIconItem not required (there might be cases, where I just want a blank instance
• support Borders (based on AnimatedIconItem color/ additional property)
• support other Shapes (I remembered that we used CircleAvatar, which is weird to begin with 😄)
• add other animation effects, apart of the rotation effect (eg. Checkbox effect)
• add a onChanged callback on the AnimatedIconButton

I will open a PR once I find the time, just now I am quite busy 😅.

[Frezyx]

Ok bro 👍🏻
This package always open for contribute