Penman serialization

[goodmami]

We now have a Penman module available (https://github.com/goodmami/penman) for serializing from arbitrary (but connected) triple lists, so it's a good time to add Penman serialization as an option for MRS.

It should be available for DMRS and EDS, particularly, but it could also be added for MRS (it's just an uglier graph). This could be done by giving the Mrs, Dmrs, and Eds classes to_triples() and from_triples() methods (like to_dict() and from_dict()). The penman module could then serialize them.

Thus, as the JSON serialization option requires two calls:

json.dumps(Mrs.to_dict(x))

The Penman serialization would also require two:

penman.dumps(Mrs.to_triples(x))