Remove MetricResult.config.

[odashi]

Related to #491

This change removes MetricResult.config. This member is not used at this point, and we basically don't need to store the same config in this struct because we can obtain the same information directly from Metric.

[neubig]

Quick clarification: When writing out the results of an analysis, I believe only MetricResult is serialized, not Metric. How would it be possible to get the information about Metric when loading in the results from a serialized file?

[odashi]

I think MetricConfig (or Metric) should be serialized separately as well. I also think parallel structure is more maintainable than nested structure.

"metric_configs": {
  "foo": <foo_config>,
  "bar": <bar_config>
},
"metric_results": {
  "foo": <foo_result>,
  "bar": <bar_result>
}

[neubig]

OK, sounds good. I'll review this.