Add success variant to button component

[aptcreativedesign]

This PR adds the variant "success" to buttons.

<flux:button type="submit" variant="success">Success</flux:button>

I understand that the "primary" variant is what is usually a success/confirmation.
However there have been times when I felt that the visual cue of a green button would be helpful to the user

[gustavosobrinho01]

This PR is very important, I will definitely use it.

[calebporzio]

Thanks for the PR! I'm pretty protective of the button variants and keeping them slim. This would create a precedent I'm not sure I'm willing to support for other components and also might have styling implications for other theming. Going to close for now. Sry, thanks.