v2.4.1

Pay careful attention to the escaping fixes in this release; the escape filter now won't double-escape strings that have already been escaped / marked safe. This matches the Jinja2 behavior.

• Don't double-escape. Thanks legutierr. Merge of
  #701.
• Prevent filter.escape from escaping SafeString. Thanks atian25. Merge of
  #623.
• Throw an error if a block is defined multiple times. Refs
  #696.
• Officially recommend the .njk extension. Thanks David Kebler. Merge of
  #691.
• Allow block-set to wrap an inheritance block. Unreported; fixed as a side
  effect of the fix for #576.
• Fix filter tag with non-trivial contents. Thanks Stefan Cruz and Fabien
  Franzen for report and investigation, Jan Oopkaup for failing tests. Fixes
  #576.