feat: add color variables into icon

[angelyaaaaa]

Purpose

• Add color variables into each icon, making user able to customize their icon colors
• Arrange those color, make it easy to change as theme

[JorgenVatle]

Thank you, @angelyaaaaa! This is looking really good. 🙌

Just spotted a minor issue with the checkmark once the animation finishes. We're missing a circle.
You can see the preview for this pull request here: https://deploy-preview-5--vue-sweetalert-icons.netlify.app/

If you have the time to correct that for me, I'd be more than happy to get your pull request merged. 😊

[angelyaaaaa]

@JorgenVatle Thanks for your response, I think I fix the problem and update the readme as well.

[JorgenVatle]

Thank you! 🙌