Markdown reader: Parse span, div tags as Span, Div elements.

Assuming markdown_in_html extension is set.
Adjusted writers and tests for change in parsing of div/span.

Textile, MediaWiki, Markdown, Org, RST will emit raw HTML div tags for divs.
Otherwise Div and Span are "transparent" block containers.
Removed scripts directory.

This has been put in its own github repo:
https://github.com/jgm/pandoc-filters-python