Lektor 2.0

• Added _discoverable system field which controls if a page should show
  up in children. The default is that a page is discoverable. Setting it
  to False means in practical terms that someone needs to know the URL as
  all collection operations will not return it.
• Added for_page function to pagination that returns the pagiantion for a
  specific page.
• Make pagination next_page and prev_page be None on the edges.
• Allow plugins to provide publishers.
• Added |markdown filter.
• Added French translations.
• Unicode filenames as final build artifacts are now explicitly disallowed.
• Serve up a 404.html as an error page in the dev server.
• Improvements to the path normalization and alt handling. This should support
  URL generation in more complex cases between alts now.
• Show a clearer error message when URL generation fails because a source
  object is virtual (does not have a path).
• Empty text is now still valid markdown.
• Lektor clean now loads the plugins as well.
• Basic support for type customization.
• Fields that are absent in a content file from an alternative are now pulled
  from the primary content file.
• Development server now resolves index.html for assets as well.
• Markdown processing now correctly adjusts links relative to where the
  rendered output is rendered.
• Added Dutch translations.
• Added Record.get_siblings()
• Added various utilties: build_url, join_path, parse_path
• Added support for virtual paths and made pagination work with it.
• Added support for Query.distinct
• Add support for pagination url resolving on root URL.
• Server information can now also contain extra key/value pairs that
  can be used by publishers to affect the processing.
• The thumbnails will now always have the correct width and height set
  as an attribute.
• added datetime type
• added support for the process_image utility functions so that plugins
  can use it directly.
• added support for included_assets and excluded_assets in the project file.
• added Spanish translations.
• added Japanese translations.
• added support for discovering existing alts of sources.
• added support for image cropping.
• added preliminary support for publishing on windows.
• children and attachments can now have a hidden flag configured explicitly.
  Attachments will also no longer inherit the hidden flag of the parent
  record as that is not a sensible default.
• changed internal sqlite consistency mode to improve performance on HDDs.
• allow SVG files to be treated as images. This is something that does not
  work in all situations yet (in particular thumbnailing does not actually
  do anything for those)