Add bridging subtypes to conllu output

[amir-zeldes]

See the syntax suggested by @martinpopel here

[martinpopel]

The CorefUD repo is currently private, so I copy here the relevant part:
Bridging (labelled) relations can be added this way: Bridge=173<188:subset,174<188:part.
The relation label is optional, so that Bridge=173<188,174<188 is still allowed. Udapi interprets then the label as _, but during serialization this "empty value" is omitted, so we get the same output as input.