tagify: working around contenteditable cursor

In order to correctly position the cursor in a contenteditable element,
the browsers display the cursor only when there is an element to
properly flow inside the contenteditable.

For Firefox, this is a <br type="_moz">. This <br> sticks to the end of
the contenteditable and does not disappear. For webkits, this is
likewise a <br>, but as you start typing, the <br> will become replaced
with the TextNode containing your text content. We then rely on input
event to check to see if we need to manually inject a <br>.

Finally upon creation of a tag, we focus on the <br> so as to position
the cursor correctly.
Merge pull request #31 from behance/tagify-cursor

tagify: working around contenteditable cursor