Add support for for selection of metadata to -m

[tovrstra]

This fixes at least a part of #98. (See issue for more details. The -m option has been extended, -M is not included.)

@psychemedia Could you try if this also works for you? Thanks! I've also updated the README.md with some practical instructions.

[codecov[bot]]

Codecov Report

Base: 98.07% // Head: 98.20% // Increases project coverage by +0.12% :tada:

Coverage data is based on head (37766b5) compared to base (5ddf4f7). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #159 +/- ## ========================================== + Coverage 98.07% 98.20% +0.12% ========================================== Files 3 3 Lines 156 167 +11 ========================================== + Hits 153 164 +11 Misses 3 3 ``` | [Impacted Files](https://codecov.io/gh/srstevenson/nb-clean/pull/159?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Scott+Stevenson) | Coverage Δ | | |---|---|---| | [src/nb\_clean/cli.py](https://codecov.io/gh/srstevenson/nb-clean/pull/159/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Scott+Stevenson#diff-c3JjL25iX2NsZWFuL2NsaS5weQ==) | `100.00% <ø> (ø)` | | | [src/nb\_clean/\_\_init\_\_.py](https://codecov.io/gh/srstevenson/nb-clean/pull/159/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Scott+Stevenson#diff-c3JjL25iX2NsZWFuL19faW5pdF9fLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Scott+Stevenson). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Scott+Stevenson)

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[tovrstra]

@srstevenson Thanks for the review. I should have caught these, but overlooked them somehow.

I'll go through the final diff to see whether I can spot anything else.

[tovrstra]

All comments should be addressed, but if something else pops up, I'm happy to fix more.

[srstevenson]

Thanks @tovrstra! I merged locally to resolve the conflicts.

[srstevenson]

This has been included in nb-clean 2.4.0, which is available from PyPI.

[tovrstra]

Thanks for taking care and making the release! We'll start using it right away.