Add node/edge attribute inspection on graph display
Use a better ES6 shim

This fixes overwriting Map entries, which used to break the internals
and thus break the Map horribly.
Add node attrib dict getter
Add edge attrib getter
Fix setting edge color through graph display
Make label setting through graph display consistent
Fix duplicate permitted node/edge attributes on save/load
Clarify error on getting attrib of nonexistent node/edge
Merge pull request #184 from Muon/master

Node/edge attribute dict getters, inspection, fixes