Anchors at the beginning of unordered list items cause errors

[mrossinek]

Currently, it is not possible to place an anchor at the beginning of an unordered list item, since the scanner attempts to parse this as a TODO item and breaks.
A simple example:

- [anchor]

[anchor]{https://github.com}

produces:

(document [0, 0] - [3, 0]
  (generic_list [0, 0] - [0, 9]
    (unordered_list1 [0, 0] - [0, 9]
      (unordered_list1_prefix [0, 0] - [0, 2])
      (ERROR [0, 2] - [0, 3]
        (ERROR [0, 2] - [0, 3]))
      content: (paragraph [0, 3] - [0, 9]
        (paragraph_segment [0, 3] - [0, 9]))))
  (ERROR [0, 9] - [0, 10])
  (paragraph [2, 0] - [2, 28]
    (paragraph_segment [2, 0] - [2, 28]
      (anchor_definition [2, 0] - [2, 28]
        (link_description [2, 0] - [2, 8]
          text: (paragraph_segment [2, 1] - [2, 7]))
        (link_location [2, 8] - [2, 28]
          type: (link_target_url [2, 9] - [2, 9])
          text: (paragraph_segment [2, 9] - [2, 27]))))))