Allow space in FORM and LEMMA in loose parsing mode

[spyysalo]

CoNLL-U version 2 allows space in FORM and LEMMA (http://universaldependencies.org/format). It may not be possible to fully reliably support this in loose parsing mode (which allows any space as field separator), but some heuristics should be impemented for basic cases.

Related to #15.

[fginter]

Maybe two spaces?