enw_metadata() target_date default be detected from the function specification

[seabbs]

This PR closes #212 by detecting the defaults for target_date from the function specification using formals().

[codecov]

Codecov Report

Merging #232 (ec87e5a) into develop (c5029b6) will not change coverage.
The diff coverage is 100.00%.

❗ Current head ec87e5a differs from pull request most recent head 760aeea. Consider uploading reports for the commit 760aeea to get more accurate results

@@           Coverage Diff            @@
##           develop     #232   +/-   ##
========================================
  Coverage    97.31%   97.31%           
========================================
  Files           14       14           
  Lines         1527     1527           
========================================
  Hits          1486     1486           
  Misses          41       41           

Impacted Files	Coverage Δ
R/preprocess.R	96.44% <100.00%> (ø)

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[seabbs]

I've self-reviewed this and planning to merge without further review as this is a relatively simple change.