one-liner statement everywhere

[the-liquid-metal]

one-liner statement is more attractive than multi-liner, and closer to jquery style.
also:

• remove parentheses in closure param if possible in order to remove distraction.
• convert function to bare statemant.
• use ternary operator
• more pragmatic example

All previous siblings (bare statemant):
[...el.parentNode.children].filter((item, idx, self) => idx < self.indexOf(el) && item.matches($selector))

All next siblings (bare statemant):
[...el.parentNode.children].filter((item, idx, self) => idx > self.indexOf(el) && item.matches($selector))

FadeToggle (ternary operator):
el.style.opacity = (opacity === "1") ? "0" : "1";

isEmptyObject (a pragmatic way):

// usage in IF statement
if (Object.keys(obj).length) {
    // ...
}

note: in most case isEmptyObject is used in IF statement. Object.keys returns array. array.length is always number. 0 is converted to false, otherwise true. there is no need to create function in order to replace isEmptyObject.

[camsong]

Looks good, could you make a PR? You can add native one-liner style to part of the statement.