Added discover_missing_links

It currently does nothing.
Added comments and new option
Created tests for issue #18
Replaced an invalid option

The option to create hanging list indents didn’t make sense since when
such formatting is needed the source Markdown wouldn’t have been valid
anyway. It’s been replaced with justifying ordered list numbering.
Added passing default link definition to util
Implemented discover_missing_links
Fixed issue where last line matched too often
Added discovering missing link definitions