Fix crash in markdown-in-html extra (#565)

[Crozzers]

This PR closes #565 by fixing a bug where an HTML snippet inside of a list could cause the library to crash when markdown-in-html is enabled.

The issue is due to how _markdown_in_html handles indentation.

- one two
   <p markdown="1">
  def
  </p>
- three four

In the above snippet, the HTML is placed inside a list because _hash_html_blocks doesn't work in lists (see #514), which means the snippet gets passed to _do_markdown_in_html.
The text is then dedented using _uniform_outdent which removes the smallest common leading indentation. Since the opening tag is indented further than the rest, it is left with a slight indentation.

The block is then passed to _hash_html_block_sub where multiple regular expressions expect the tag to be right at the start of the line. This causes a match to return None and the library fails to get the first match group, causing the crash.

[nicholasserra]

Thank you!