Correctly handle numbers for ordered list items

[rpaul-stripe]

This PR addresses issue #371, ensuring that ordered lists are rendered with a start attribute that reflects the number of the first item in the list. This is consistent with the behavior described in the CommonMark specification. The PR also introduces a new value attribute on list item AST nodes that captures the list number used for each ordered list item.

• Parser modifications
  • Modifies the parser so that the leading symbol for a list item is assigned to the value property on the AST node
  • Adds a parser Jasmine test case to ensure that ordered and unordered lists have the correct values for the value attribute
• Schema modifications
  • Modifies the schema to add the new value attribute to the list item node
  • Modifies the list node transform function so that when the list is ordered it walks the children, obtains the value attribute of the first child, and uses that as the start value passed into the render tree
  • Modifies a marktest case with ordered list items in order to reflect the presence of the start attribute in the render tree
• Formatter modifications
  • Modifies the formatter so that it computes the list prefix for each individual item, using the value attribute for ordered list items and falling back to "1" when it is not present
  • Modifies the formatter test cases so that they verify that the ordered list numbers are preserved

Closes #371

[mfix-stripe]

Thanks for handling @rpaul-stripe!