implement lists

[bantic]

A list section is different from other markup sections because it has sub-elements that are not markers.

A normal markup section has (a linked-list of) markers:
markupSection -> markers

But a list section has list items which have markers:
listSection -> listItems -> markers

To accommodate lists, the Abstract Tree (AT) will need to be able to traverse the leaf nodes (markers) of the tree — for semantic actions like splitting/joining sections and markers — without assuming that the max depth of children is always going to be 1.

• editor-dom renderer can render list sections and list items
• mobiledoc parser can parse list sections and list items
• mobiledoc dom renderer can render list sections and items
• mobiledoc html renderer can render list sections and items
• Post parser must be able to reparse list sections
• Actions on the post editor like splitMarkers must be able to handle markers that have a depth > 1

Creating a list will happen the same way that other section types are created: Click a toolbar button. Eventually (#87) also via text-expansion.
Exiting a list happens by hitting enter when the cursor is in the last item in a list and it is empty (this is how google docs works, too).

For now, only singly-nested lists (this is how medium does it).

[bantic]

WIP on the lists branch