New feature: empty rulesets analysis

When I was scrolling to some of my competitors’ websites, I noticed that analyze-css by Maciej Brencz was counting the amount of empty rules in your CSS. It seemed to me that this was easy to add to Wallace, so I did it in a short amount of time. Here is the explanation that will eventually end up in the docs in some form:

A rule (or ruleset) is considered empty when it contains no declarations. Whitespace is ignored when determining whether a rule is empty or not.

Examples

/* This is an empty rule */
.header {
}
/* This is also an empty rule */
.header {
}
/* This is not an empty rule */
.header {
	margin: 10px;
}

Please note that at-rules are not part of this analysis, so an empty media query rule is not counted here. That might be a future improvement.

Finally, a link to the commit that contains this change.