Support other metadata merging strategies

Currently metadata files are combined with an inner join, which means that any records not present in all metadata files will be dropped. It would be useful if this could be a configurable parameter.

The use-case is to visualize feature importances in Empress together with taxonomy data, and other feature-level annotations. When features are filtered, this results in the dataset shrinking in size to only include the shared set.

cc @thermokarst @andrewsanchez @ebolyen @fedarko

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