CoNLL-U v2 support

[spyysalo]

The UD v2 version of the CoNLL-U spec extends the v1 version implemented by conllu.js in at least the following

• Space is permitted in FORM and LEMMA
• Empty nodes are allowed in the extended representation

Details: http://universaldependencies.org/format.html

These features should be supported.

[spyysalo]

The following example is now accepted, but the empty node is not shown:

# visual-style 6 7 obj color:red
# visual-style 5.1 5 nsubj color:red
# visual-style 2 5.1 conj color:red
# visual-style 5 6 remnant color:blue
# visual-style 2 5 conj color:blue
1   I    _ _ _ _ 2 nsubj   _  _
2   like _ _ _ _ 0 root    _  _
3   tea  _ _ _ _ 2 obj     _  _
4   and  _ _ _ _ 5 cc      _  _
5   you  _ _ _ _ 2 conj    5.1:nsubj  _
5.1 E5.1 _ _ _ _ _ _       2:conj _
6   rum  _ _ _ _ 5 remnant 5.1:obj _
7   .    _ _ _ _ 2 punct   _  _