Add download option to skip accession numbers

[spolcyn]

This improves download efficiency when some filings have been already downloaded to another location, such as when doing incremental updates.

The existing logic to skip already downloaded files when they are in the same download_folder is useful for local-only use, but is less useful when files are only local for a brief period before being sent to long-term storage.

[spolcyn]

@jadchaar This feature has been manually tested -- would love a quick sanity check that this feature is reasonable to add and that I didn't miss it somewhere. Then will add automated tests.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (4e5d100) to head (b9aa251). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #142 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 8 8 Lines 220 223 +3 Branches 30 32 +2 ========================================= + Hits 220 223 +3 ```

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[spolcyn]

I have applied the suggestions and added a test. Let me know if there are any outstanding issues!

[spolcyn]

@jadchaar Any further issues or clarifications with this PR?

[jadchaar]

Thanks again for the contribution @spolcyn. LGTM!