classList is not support by IE < 10

[wilsoncook]

line 432:

    // set default class
    editor.classList.add(defaults.class);

the attribute classList is not exist under IE < 10, actually it's not necessary to use this attribute, instead there are more convenient methods to browser compatibility, for example className, also see https://developer.mozilla.org/en-US/docs/Web/API/Element/classList

don't know why use classList, not aiming to support IE < 10 from the first thought?