metrics: introduce diff

[efiop]

This first implementation is based on existing dvc metrics show
functionality, which has type and xpath support to determine
specific values in your file that you are interested in. That makes
comparing pretty straightforward to implement.

Related to #2995

TODO:

• Possibly multiple fields in the metrics file Multiple metrics in one file #1682 to match what has been described in the original issue.
• Reconsider CLI flags (e.g. old-rev/new-rev are not the nicest looking). UPDATE: for now following the same scheme as in dvc diff
• Probably better to show the key for a specific metric field in the output, but that doesn't really match up with our current metrics show behaviour in case of type/xpath being specified.
• If type/spath are not specified, we could detect the type and diff the whole csv/etc/json. UPDATE: for now keeping it explicit, same as in dvc metrics show.

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iterative/dvc.org#921

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[efiop]

@dmpetrov asked for this format specifically, so don't be surprised that we don't show old value here :)