Fix and improve literal mode.

  * Require a blank line as delimiter between text and code.
  * Don't ignore indentation with mixed whitespace.
  * Handle blank lines depending on preceding lines.
    (Addresses changes in #190.)
  * Bonus: Remove unused variable (leftover from #190).
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Fix Wikipedia link by escaping parens
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Merge pull request #195 from rgieseke/master

Minor documentation and link fixes
Merge pull request #194 from marchaefner/master

Fix and improve literal mode.