Implement plugin 'publish' feature

Implemented:

Currently supported output formats for local_file:

Changes on specs:

No separate stdout plugin publisher. By default render cli command prints to stdout.
Adding --publish flag triggers all publish blocks defined in fabric template. Returns an error if there is no publish block defined for document.
--format instead of --publish-format. --format overrides default output format. If format is defined in the fabric template, then it uses the one from template.
format instead of doc_format in publish block arguments.
No duplicate format_ext in data context since it already matches format.
By default html <title> is from frontmatter as described in specs, but if not found then the first title from the content is used. If document has no title elements, then Untitled is used.

Incomplete:

Frontmatter css_classes are quite tricky to implement for html output format. I started working on this but it requires writing custom goldmark renderer which was quite buggy on first iteration, so I removed this functionality for now. css_code, css_sources and all other html properties from frontmatter works fine.

Example using publish:

blackstork-io / fabric