fix for creating new list items

[Leshkens]

#27

[kWeb24]

This fix works in Firefox but not in Chrome. I found out that li elements must be contenteditable="true". I added this line at the end of _make() method:

if (tagName === 'li') el.setAttribute('contenteditable', true);

This did the trick but should rather be moved to the attributes parameter and set when the method is called.