Support CoNLL-U Plus as input and output format

[chiarcos]

Support for a recently suggested extension of the CoNLL-U format: https://universaldependencies.org/ext-format.html

Note that this format allows to read TSV data without providing explicit column names (as these are stored in the data). Thus requires a specialized reader module.

[cfaeth]

Has been handled in #17
All comments are now stored on sentence-level. global.columns is used as default for naming of conll:COL properties. (Can still be overwritten by command-line parameters cf. README.md)

Still open: handling of other relevant fields:
sent_id
text
text_{lang}