Allow arbitrary choice of scoring metric

[berkgercek]

In principle there is no constraint on the choice of metric one uses for the nglm.score() method. There are a present number of implemented choices, but accepting a callable should be an option as well. Additionally, the existing options need more documentation on how they are used.

Finally, there should be a confirmation for the builtin metrics that the user is using the right metric for their model link function, optionally throwing a warning.