dvc plots: allow for setting output directory via config

[ykasimov]

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• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

This PR allows users to set output directory for dvc plots show via dvc config. I added a test covering this scenario.
To test run the following script:

#!/bin/bash

set -ex
rm -rf repo
mkdir repo
pushd repo

echo '[{"first_val": 100, "val": 2}, {"first_val": 200, "val": 3}]' >> metric.json

dvc config plots.out_dir config_dir

dvc plots show metric.json

dvc plots show -o arg_dir metric.json

[pared]

Looks good, some minor things related to tests.

[daavoo]

P̶e̶r̶h̶a̶p̶s̶ ̶I̶ ̶a̶m̶ ̶o̶v̶e̶r̶t̶h̶i̶n̶k̶i̶n̶g̶ ̶a̶n̶d̶ ̶i̶t̶'̶s̶ ̶j̶u̶s̶t̶ ̶a̶ ̶m̶a̶t̶t̶e̶r̶ ̶o̶f̶ ̶a̶d̶d̶i̶n̶g̶ ̶e̶x̶a̶m̶p̶l̶e̶s̶ ̶i̶n̶ ̶d̶o̶c̶s̶.̶ ̶S̶t̶i̶l̶l̶,̶ ̶w̶e̶ ̶a̶l̶l̶o̶w̶ ̶s̶t̶o̶r̶i̶n̶g̶ ̶a̶n̶ ̶u̶n̶r̶e̶s̶o̶l̶v̶e̶d̶ ̶p̶a̶t̶h̶ ̶i̶n̶ ̶̶.̶d̶v̶c̶/̶c̶o̶n̶f̶i̶g̶̶ ̶s̶o̶ ̶u̶s̶e̶r̶s̶ ̶c̶a̶l̶l̶i̶n̶g̶ ̶f̶r̶o̶m̶ ̶d̶i̶f̶f̶e̶r̶e̶n̶t̶ ̶C̶W̶D̶ ̶w̶i̶l̶l̶ ̶s̶a̶v̶e̶ ̶p̶l̶o̶t̶s̶ ̶t̶o̶ ̶d̶i̶f̶f̶e̶r̶e̶n̶t̶ ̶f̶o̶l̶d̶e̶r̶s̶.̶

Looks like we already do this for other path config options (i.e. html template path)

[pared]

@ykasimov @daavoo has rebased the PR, could you squash the commits and include which issue it fixes in the commit message?

[ykasimov]

All done. 👍