add support for conllu format --conllu-format

[ftyers]

This patch allows for using the command line option --conllu-format "true" to output in CoNLL-U format[1], preserving comments, segments and empty nodes.

It also allows for training data to be in CoNLL-U format, regardless of output format.

It should provide the same output as udpipe --tag

On the TODO list is allowing input like:

# sent_id = es-dev-001-s15
# text = Tan solo Lemke puede retirar la maldición, y Billy trata de persuadir al anciano de que la retire.
1	Tan	_	_	_	_	_	_	_	_
2	solo	_	_	_	_	_	_	_	_
3	Lemke	_	_	_	_	_	_	_	_
4	puede	_	_	_	_	_	_	_	_
5	retirar	_	_	_	_	_	_	_	_
6	la	_	_	_	_	_	_	_	_
7	maldición	_	_	_	_	_	_	_	_
8	,	_	_	_	_	_	_	_	_
9	y	_	_	_	_	_	_	_	_
10	Billy	_	_	_	_	_	_	_	_
11	trata	_	_	_	_	_	_	_	_
12	de	_	_	_	_	_	_	_	_
13	persuadir	_	_	_	_	_	_	_	_
14-15	al	_	_	_	_	_	_	_	_
14	a	_	_	_	_	_	_	_	_
15	el	_	_	_	_	_	_	_	_
16	anciano	_	_	_	_	_	_	_	_
17	de	_	_	_	_	_	_	_	_
18	que	_	_	_	_	_	_	_	_
19	la	_	_	_	_	_	_	_	_
20	retire	_	_	_	_	_	_	_	_
21	.	_	_	_	_	_	_	_	_

This fixes issue #6

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

[aryamccarthy]

Bumping because this is super useful.