Link tooltip doesn't appear if under bottom bound of editor

[ygrishajev]

I faced the issue of link tooltip not appearing when bounds option is set and selection is the most bottom line of the editor. In this case tooltip is out of the bounds.

Steps for Reproduction

1. Visit http://beta.quilljs.com/
2. Enter any text in the very bottom of the demo editor
   
3. Try to create a link of this text

Expected behavior: Tooltip appears above the selection (as a suggestion)

Actual behavior: Tooltip doesn't appear

Platforms: Chrome 48 on Mac 10.11

Version: 1.0.0-beta.11

This may be implemented with additional check/adjustment here https://github.com/quilljs/quill/blob/develop/ui/tooltip.js#L40

if (rootBounds.bottom > containerBounds.bottom) {
    const shift = containerBounds.bottom - rootBounds.bottom - this.root.offsetHeight
    this.root.style.top = `${top + shift}px`
  }