sub menu support

add an option to include a sub menu below the main menu. a sub menu is in use at https://ajp.fm/, but there it is hard coded for some pages. not an ideal solution.

• check if the header_last config setting can be used.
• consider adding a new field, where one can add html code.
• consider adding a new field to all .md files (eg. include_first), which can link to an internal file that will be included before the rest.