Release 3.8

Changed

• DRY fix in abbr extension by introducing method create_element (#1483).
• Clean up test directory by removing some redundant tests and port
  non-redundant cases to the newer test framework.
• Improved performance of the raw HTML post-processor (#1510).

Fixed

• Backslash Unescape IDs set via attr_list on toc (#1493).
• Ensure md_in_html processes content inside "markdown" blocks as they are
  parsed outside of "markdown" blocks to keep things more consistent for
  third-party extensions (#1503).
• md_in_html handle tags within inline code blocks better (#1075).
• md_in_html fix handling of one-liner block HTML handling (#1074).
• Ensure <center> is treated like a block-level element (#1481).
• Ensure that abbr extension respects AtomicString and does not process
  perceived abbreviations in these strings (#1512).
• Ensure smarty extension correctly renders nested closing quotes (#1514).