* Added: three hooks `register_variables/macros/filters` to facilitate
cooperation with other MkDocs plugins.
* Fixed: `define_env() was always required in module (191)
* Added: trace the case when no module is found (INFO)
* Improved documentation, particularly about HTML pages
* Added: parameters `j2_comment_start_string` and
`j2_comment_end_string` to plugin's parameters,
to specify alternate markers for comments.
* Added the multiline parameter `force_render_paths` in the config file,
to specify directories or file patterns to be rendered for the case when `render_by_default = false`
(the `render_macros` parameter in the YAML header of the page
has the last word).