Add metadata from scorer result

[David Elner (delner)]

Addresses #115

This pull request allows users to return a Hash as a result with :metadata; this field will be used as the metadata for the scorer span.

Example:

Braintrust::Scorer.new("translation") do |expected:, output:|
  common_words = output.downcase.split & expected.downcase.split
  overlap = common_words.size.to_f / expected.split.size
  {
    score: overlap,
    metadata: {word_overlap: common_words.size, missing_words: expected.downcase.split - output.downcase.split}
  }
end