Fix enter behavior when the next list item is done

[gergesh]

Previously, if a list item was followed by a done item there was no
way to insert an additional one.

In the long term, I think the current behavior is still clunky and the one
employed by Keep (which moves all done items to a separate section)
is more intuitive (but perhaps best kept under a toggle).

[OmGodse]

I think you're partially correct, the current behavior to do nothing when the next list item is done has to be changed.

However, I don't think that adding a list item if the next is done is the correct model. I'll modify it such that if the next list item is checked, the cursor would jump to the one after that and so on, until it encounters the end of the list, at which point, a new item would be inserted