Create log_snakemake() function

[kelly-sovacool]

Change(s) made

• Created log_snakemake(), which redirects output to a log file when it is called from a Snakemake rule and a rule-specific log file is defined.
• Wrote a unit test to check that log_snakemake() reports that no log file exists if that's the case and quiet = FALSE. However, it doesn't seem possible to test that it creates a log file with sink(), as testthat seems to prevent that from happening. It does work when run from the console.

Checklist

(Strikethrough any points that are not applicable.)

• Write unit tests for any new functionality.
• Update roxygen comments & vignettes if there are any API changes.
• Update NEWS.md if this includes any user-facing changes.
• The check workflow succeeds on your most recent commit.

[sklucas]

Seems like a really useful function!

[kelly-sovacool]

@sklucas I have been copy-pasting it into my Snakemake + R projects so often that it became absurd for it to not be in a package somewhere. Many thanks!