Add support for CoNLL-U framework

[Hinrik Hafsteinsson]

Language
CoNLL-U is an annotation framework for syntactically annotated treebank files in the Universal Dependencies project, which contains 200+ datasets for 100+ languages.

The framework is relaitvely simple, so syntax highlighting should be easy to configure for it.

Additional resources

Example CoNLL-U content:

# sent_id = 1
# text = They buy and sell books.
1   They     they    PRON    PRP    Case=Nom|Number=Plur               2   nsubj   2:nsubj|4:nsubj   _
2   buy      buy     VERB    VBP    Number=Plur|Person=3|Tense=Pres    0   root    0:root            _
3   and      and     CCONJ   CC     _                                  4   cc      4:cc              _
4   sell     sell    VERB    VBP    Number=Plur|Person=3|Tense=Pres    2   conj    0:root|2:conj     _
5   books    book    NOUN    NNS    Number=Plur                        2   obj     2:obj|4:obj       SpaceAfter=No
6   .        .       PUNCT   .      _                                  2   punct   2:punct           _

# sent_id = 2
# text = I have no clue.
1   I       I       PRON    PRP   Case=Nom|Number=Sing|Person=1     2   nsubj   _   _
2   have    have    VERB    VBP   Number=Sing|Person=1|Tense=Pres   0   root    _   _
3   no      no      DET     DT    PronType=Neg                      4   det     _   _
4   clue    clue    NOUN    NN    Number=Sing                       2   obj     _   SpaceAfter=No
5   .       .       PUNCT   .     _                                 2   punct   _   _

Official documentation:
UD home page

Existing highlighting implementations for other environments:
VSCode
SublimeText
Atom (RIP)