Add accuracy metric to crows-pairs (#380)

* add accuracy metric to crows-pairs

tests/testdata/crows_pairs_french_religion-v0-res.json

-{"results": {"crows_pairs_french_religion": {"likelihood_difference": 0.32691651640972225, "likelihood_difference_stderr": 0.021833493193249474}}, "versions": {"crows_pairs_french_religion": 0}}
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+{"results": {"crows_pairs_french_religion": {"likelihood_difference": 0.32691651640972225, "likelihood_difference_stderr": 0.021833493193249474, "pct_stereotype": 0.45217391304347826, "pct_stereotype_stderr": 0.046614569799583463}}, "versions": {"crows_pairs_french_religion": 0}}
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tests/testdata/crows_pairs_french_sexual_orientation-v0-res.json

-{"results": {"crows_pairs_french_sexual_orientation": {"likelihood_difference": 0.3160680928470684, "likelihood_difference_stderr": 0.02397758321605678}}, "versions": {"crows_pairs_french_sexual_orientation": 0}}
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+{"results": {"crows_pairs_french_sexual_orientation": {"likelihood_difference": 0.3160680928470684, "likelihood_difference_stderr": 0.02397758321605678, "pct_stereotype": 0.43956043956043955, "pct_stereotype_stderr": 0.05231815698566189}}, "versions": {"crows_pairs_french_sexual_orientation": 0}}
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tests/testdata/crows_pairs_french_socioeconomic-v0-res.json

-{"results": {"crows_pairs_french_socioeconomic": {"likelihood_difference": 0.3394681494647815, "likelihood_difference_stderr": 0.01702488895584347}}, "versions": {"crows_pairs_french_socioeconomic": 0}}
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+{"results": {"crows_pairs_french_socioeconomic": {"likelihood_difference": 0.3394681494647815, "likelihood_difference_stderr": 0.01702488895584347, "pct_stereotype": 0.4642857142857143, "pct_stereotype_stderr": 0.035714285714285705}}, "versions": {"crows_pairs_french_socioeconomic": 0}}
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