v0.21

• a lot off code changes. Many regarding hyphens
• added an extract_final_hyphen function, used by several programs
  FoLiA-abby, FoLiA-page and FoLiA-txt
• FoLiA-txt: filter out ZWNJ characters. Avoid spurious LineBreaks
• fix for #68
• FoLiA-idf was not quite working. Fixed
• FoliA-page: removed --sent option. updated man page
  lots of other fixes too
• added experimental FoLiA-merge program:
  merging lemma/pos information into FoLiA files
• more and better tests added
• updates in README.MD