[egf]: allow untyped links

[nichtich]

In lack of a formal specification It's not strictly forbidden at the moment. Explicit mentioning the possibility of untyped links would help:

foo
  #ref bar

Or with https://github.com/thi-ng/umbrella/issues/234

foo
  -> bar

The parser could treat untyped links as links with property label being the empty string.

[postspectacular]

I'm not quite sure how you'd represent these in the target environment/datastructure. E.g. in JS each node is an object with properties and their values as KV pairs. So for untyped edges between nodes we'd need to introduce another special property (like $id or $ref). Not sure what to call that, really...