Created by: tagliala

Hi,

I know this is a quite invasive PR and is highly opinionated.

I've noticed some inconsistent styling in the sources.

The use of empty line between blocks was quite inconsistent, this linter will increase the uniformity of the code.

Examples:

• badge has empty lines between colored variants https://github.com/twbs/bootstrap/blob/v4-dev/scss/_badge.scss#L58 while alert hasn't https://github.com/twbs/bootstrap/blob/v4-dev/scss/_alert.scss#L47
• card has both empty lines and not empty lines between *-child selectors https://github.com/twbs/bootstrap/blob/v4-dev/scss/_card.scss#L50 https://github.com/twbs/bootstrap/blob/v4-dev/scss/_card.scss#L236
• popover has both empty lines and not empty lines between ::before and ::after selectors https://github.com/twbs/bootstrap/blob/v4-dev/scss/_popover.scss#L39 https://github.com/twbs/bootstrap/blob/v4-dev/scss/_popover.scss#L168
• dropdown has empty lines between -left and -right variants https://github.com/twbs/bootstrap/blob/v4-dev/scss/_dropdown.scss#L124 while navbar hasn't https://github.com/twbs/bootstrap/blob/v4-dev/scss/_navbar.scss#L112