Add support for TODO items:

• TODO items should show a checkbox on the side
• The checked state should be stored in the prosemirror state
• If possible, content of checked todo items should appear strikethrough (I think we can do this directly from CSS)
• show "To-do" as placeholder

We currently store a listType attribute on Blocks. On first thought, we could:
a. use listType="todo-checked" and listType="todo-unchecked"
b. use listType="todo" and todo-checked="true" as attributes

Option B seems cleaner, but option A has other benefits (we don't need to clean for "invalid" Prosemirror states, for example, what does listType="li" and todo-checked mean? In that case todo-checked should be removed. What do you think is the best architecture for this?