rename .contains() to .includes()

https://twitter.com/mathias/status/535350698918297600
https://github.com/mathiasbynens/String.prototype.includes
https://codereview.chromium.org/742963002
https://esdiscuss.org/topic/array-prototype-contains-solutions
https://github.com/mootools/mootools-core/commit/521471f5c0617edc12c680c
dad8346c3eb95776a
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Ob
jects/String/includes
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Ob
jects/Array/includes