Support list nodes with line breaks

[JimmyCushnie]

Currently, each node of a list can only have one line. A line break will be interpreted as the end of the list. In accordance with markdown spec, a single line break should be treated as a line break within the current list node.

Input:

1. List
1. More list
Second line
1. Last list

Ok now the list is over

Currently looks like:

1. List
2. More list

Second line

1. Last list

Ok now the list is over

Should look like:

1. List
2. More list
   Second line
3. Last list

Ok now the list is over

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Related: #2