Add ConceptNet

[cthoyt]

Issue #2 brought attention to the ConceptNet as a possible dataset to include with PyKEEN. They provide a tab-separated dump of the database here. It is not pre-stratified into training/testing/evaluation sets.

Because this file has additional columns besides head, relation, and tail, its inclusion will also require an updated to the SingleTabbedDataset such that the usecols keyword argument can be specified in the dataset's __init__()

Blocked by #196 because splitting algorithm is currently too slow for big datasets (with more than ~5 million triples)