Add ability to force-save notebooks to ottr::export

[chrispyles]

ucbds-infra/otter-grader#474

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (67bf57b) 99.54% compared to head (6508ed3) 99.57%.

❗ Current head 6508ed3 differs from pull request most recent head 71a4619. Consider uploading reports for the commit 71a4619 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   99.54%   99.57%   +0.03%     
==========================================
  Files          14       15       +1     
  Lines         219      238      +19     
==========================================
+ Hits          218      237      +19     
  Misses          1        1              

Impacted Files	Coverage Δ
R/export.R	97.61% <100.00%> (+0.64%)	⬆️
R/save_notebook.R	100.00% <100.00%> (ø)
R/utils.R	100.00% <100.00%> (ø)

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