v0.15.0

• Moved from pulldown-cmark to comrak for parsing Markdown. This may have some
  performances drawbacks but allows for a few more features:
  • Description lists
  • Strikethrough
  • Task items
• New option:
  • crowbook.files_mean_chapters allow to enforce that each files
    means a chapter break or to make sure that it doesn't (by default,
    only true for numbered chapters).
• Fallback on Rust zip library when there is no zip command.
• By default, don't add an empty chapter title for non numbered
  "chapters" that don't contain a title.
• Now uses reqwest instead of hyper to connect to languagetool/grammalecte.
• hyphenation dependency is now optional.
• Dependencies update.
• Fix type deduction issues for new rustc compliler