Foliantcontrib.mkdocs

- Add logs.
- Update for Foliant 1.0.4: Pass logger to spinner.
- Require Foliant 1.0.4.

- Add `use_headings` and `slug` options for MkDocs backend.
- Fix inappropriate translation of image URLs into local paths in MkDocs preprocessor.

- Add `ghp` target for GitHub Pages deploy with `mkdocs gh-deploy`.