Rule 16 uncaptured cases and false positives

[SecondSkoll]

Please see comments from @ru-fu in #52, an I've added a few additional cases.

Cases include:

• Capturing all MyST directives that contain #
• Not capturing code blocks declared by indentation (MD)
• Not capturing code blocks delimited by :: (RST)
• Not capturing indented code blocks, as the initial delimiter has a start of new line requirement (MD)
• Will capture content with # contained inline (e.g. URLs with anchors or other random strings)
• Not capturing MyST colon fenced code blocks
• Unable to parse included code files, or excerpts from code - likely out of scope, would require an additional scoping value and additional rules.