Allow evaluate CLI to take metrics from the command line arguments

Currently the evaluate CLI has fixed list of metrics which are reported as part of the evaluation output. This is limiting when one wishes to make note of other metrics/scores that are part of custom pipeline components in their evaluation outputs. My suggestions is to allow users to pass a list of metrics that they wish to add to the existing set of metrics that are part of the evaluate CLI script.

## Which page or section is this issue related to?

https://github.com/explosion/spaCy/blob/master/spacy/cli/evaluate.py#L79-L97