Allow specifying one's own file name for evaluation

[rafguns]

At the moment, the listeners in linkpred.evaluation.listeners use fixed file names (well, they change, depending on dataset, predictor and time stamp, but it's not really possible to specify your own name). That sucks.

My original thought was that base class Listener would just accept an extra argument in its ctor, which could then be used by all descendant classes. The problematic cases, however, are CachePredictionListener and CacheEvaluationListener, since they can actually generate multiple files (e.g., one per predictor). Possibilities:

• change them to deliver all their output to one file
• figure out how we'd like to handle them