Add directives into source files

[RobertDober]

A typical use case would be to add the following to a docstring

<!-- %extractly%ignore_line%
  Some markdown useful inside the extracted markdown but not in hexdocs where there is more context
 %extractly%ignore_line% -->

However sometimes it might also be a good idea not to extract all information from a docstring and therefore three more directives shall be implemented

<!-- %extractly%stop_processing% this directive ends all extraction -->
<!-- %extractly%suspend_processing% name says it all -->
<!-- %extractly%resume_processing% idem -->