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).
