add_header()
  Add email headers.
  
  This scanner can be used to add an header line to email.
  
  Usage: add_header('Header_name','header_data',scanners)
     or: add_header(...).onlyif('virname')
  
  Where: 'Header_name' is a string, which defines name of the header
         'header_data' is a string, which defines header's content
         'virname' is a string, which defines any virus name.
           By default all virus names at least 1 character long: '.'
         There are some special pattern, which are replaced:
           %V ... virus name
           %S ... scanner name
           %L ... detected level