Introduce a vaguely CONLL shaped output

The hope is that this would be

1. more compact (single edu instead of pairs, so less redundancy)
2. more recognisable to people from parsing communities
3. more standard(ish)

But note the awkwardness around the output being more a graph
than a tree
Try to document our CONLL style format a bit
Merge remote-tracking branch 'upstream/master'