Reading xref section (with all subsections)

added
  - tests
  - fn add_xref_entry
  - fn free_xref_entry
  - fn xref_init
  - fn xref_free
  - fn read_xref_table
  - fn print_xref
  - fn skip_leading_whitespaces
  - fn parse_keyword
removed
  - modes (also sigil_process -> sigil_verify)