--show-md output for metrics, params and dvc diff

[dmpetrov]

Today we support JSON output for the diff commands. However, CI/CD systems understand only the MD format. We need to add the MD format support to avoid the conversion.

[dmpetrov]

It might be related to #3435

[dmpetrov]

A bit more clarification: we are interested only in MD format with no HTML like

| Path | Metric | Value | Change|
|------|--------|-------|-------|
|summary.json|accuracy|0.853481| -0.334|

A nice option that can be included:

1. --no-path - do not show the first path column since the user might decide to output tables per file with separate headers for each of the tables.
2. --old-value (for metrics only) - to print both the old value and new value in addition to the change. Scenario - in the early stages of a project when metrics change a lot this format is more convenient.
3. --no-text - now dvc metrics diff output None and diff not supported. it might be better to have empty values instead.

[dmpetrov]

For dvc diff MD result should show the operation as a column deleted\added\modified. E.g.

Action | Path
added | dir1/file1
added | dir1/file2
deleted | file3

It would be nice to have options like --added-only \ --modified--only \ --deleted-only. It'll help the user to create separate sections in MD file if the separation is important and simplifies the table since the operation type column won't needed.

[efiop]

It would be nice to have options like --added-only \ --modified--only \ --deleted-only. It'll help the user to create separate sections in MD file if the separation is important and simplifies the table since the operation type column won't needed.

For the record: decided not to go with these, as it seems too out of scope for dvc diff. It is easy to parse from the MD table.

[efiop]

Keeping open to consider 1,2,3 above.