Clarify `excluded` parameter to `Predictor.__init__`

[rafguns]

#12 showed that the intended use of excluded can be misunderstood. As I write there:

Note that the excluded argument to a predictor (SimRank in this case) is intended to exclude certain edges from appearing in the results, not to exclude them during training.

So I think two things need to happen:

• rename this to be clearer
• document what it's for

[rafguns]

The doc string is actually fairly clear: https://github.com/rafguns/linkpred/blob/master/linkpred/predictors/base.py#L48. As it says

This is useful to, for instance, make sure that we only predict new links that are not currently in G.

Maybe a clearer (but somewhat less flexible) solution would be to revert the API to what was there many years ago: a boolean argument only_new (or a similar name).