No suggestions on IE11

[waht]

On Internet Explorer 11 horsey fails to display suggestions. The developer console shows that on line 365 parent is undefined (an exception is thrown).

364:      for (let char of text) {
365:        parent.insertBefore(spanFor(char), el);
366:      }

I found out that document.createTreeWalker as used in getTextChildren is unsupported by IE (all versions according to MDN: https://developer.mozilla.org/de/docs/Web/API/Document/createTreeWalker). This seems to lead to parent being undefined.

[korywka]

confirm. broken on ie.

[backflip]

I have opened a PR fixing IE9+: #50