Add `--mode` option

[jwodder]

Closes #43.

TO DO:

• [x] Give test-files subcommand a --save-results option
• [x] Remove deleted assets from status.yaml files when updating a single asset?
• [x] Add random-outdated-asset and/or random-outdated-asset-first mode(s)

[codecov[bot]]

Codecov Report

Patch coverage: 54.00% and project coverage change: -7.03 :warning:

Comparison is base (ed6d29a) 65.77% compared to head (0648824) 58.75%.

:exclamation: Current head 0648824 differs from pull request most recent head e5261ef. Consider uploading reports for the commit e5261ef to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #44 +/- ## ========================================== - Coverage 65.77% 58.75% -7.03% ========================================== Files 8 8 Lines 488 594 +106 Branches 114 136 +22 ========================================== + Hits 321 349 +28 - Misses 149 228 +79 + Partials 18 17 -1 ``` | [Impacted Files](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [code/src/healthstatus/tests.py](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-Y29kZS9zcmMvaGVhbHRoc3RhdHVzL3Rlc3RzLnB5) | `52.00% <0.00%> (ø)` | | | [code/src/healthstatus/checker.py](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-Y29kZS9zcmMvaGVhbHRoc3RhdHVzL2NoZWNrZXIucHk=) | `38.67% <27.64%> (-4.88%)` | :arrow_down: | | [code/src/healthstatus/\_\_main\_\_.py](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-Y29kZS9zcmMvaGVhbHRoc3RhdHVzL19fbWFpbl9fLnB5) | `61.45% <44.44%> (+5.09%)` | :arrow_up: | | [code/src/healthstatus/aioutil.py](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-Y29kZS9zcmMvaGVhbHRoc3RhdHVzL2Fpb3V0aWwucHk=) | `50.00% <50.00%> (ø)` | | | [code/src/healthstatus/core.py](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-Y29kZS9zcmMvaGVhbHRoc3RhdHVzL2NvcmUucHk=) | `75.87% <73.74%> (-19.78%)` | :arrow_down: | 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=dandi). 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=dandi)

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[yarikoptic]

Familiarized myself with diff, will test as deployed. Thank you @jwodder -- diff looks monumental ;)