* metrics: diff: make table more intuitive
Currently we have a "diff not supported" marker in the table, that is
used whenever dvc is not able to diff the values (e.g. no old one, no
new one or format error). It is quite ugly and it is much more intuitive
to just show both old and new values and now show that marker at all, as
if there is no old/new value it is intuitive that we can't calculate the
change. Next step is to add old/new "bad format"/etc markers for
unsupported types or malformed/missing metric files.
E.g., before:
```
Path Metric Value Change
metrics.json foo 1.2 diff not supported
```
after this PR:
```
Path Metric Old New Change
metrics.json foo None 1.2
```
* metrics/params: use em dash in diff table
* params/metrics: diff: implement tabulated markdown output
Via `--show-md` on `diff`s.
* table: show None on missing val
* tests: use `dedent`, add tests for markdown
* diff: add --show-md
Co-authored-by: Ruslan Kuprieiev <[email protected]>
Paweł Redzyński
Ruslan Kuprieiev
Saugat PachhaiandRuslan Kuprieiev