pygments, markdown upgrade, new extensions, and more

• Add delete, githubemoji, tasklist, headeranchor, b64, magiclink, and github extensions
• Upgrade to latest Markdown 2.4.1 package that includes new smarty extension
• Code highlighting is now done with pygments (highlight.js has been discontinued)
• Remove markdown2 parser (markdown2 has been discontinued)
• Gracefully handle a bad extension name
• Improvements to absolute path replacement
• Use of absolute path replacement post-processor is now optional on files for all parsers
• Relative images can can be converted to absolute path, base64, or left alone for all parsers
• Experimental optional critic markup striping pre-processor for all parsers
• Language guessing can be turned off for the highlighter
• Improved default CSS file
• Reduce commands in the command palette
• Most plugins enabled by default now (can be disabled)
• Internal bug fixes and cleanup