v5: Variable for button hover contrast

Created by: codiak

https://github.com/twbs/bootstrap/blob/a1708ec2a38d331c4c9eacac08ce9c9839649476/scss/mixins/_buttons.scss#L10

Howdy! Would it be possible to either darken or accept a custom variable for the button hover contrast?

For the meantime, we can override the "button-variant" mixin, but it seemed like an element others might find useful to adjust as well.

Thanks!