Log Message:
sfeed: Update to 0.9.18

Changes:
0.9.18
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 - format tools: don't skip items with a missing/invalid timestamp field
 - sfeed_atom: the updated field is mandatory: use the current time
 - sfeed_atom: fix timezone, output if timestamp is set
 - sfeed_atom: make the output more conform by adding id, updated tags and add
   <feed> tags (author name, id, updated, title).
   Thanks lich for the feedback and testing.
 - sfeed_gopher: do not use URL: prefix for gopher:// urls.
 - Documentation improvements