implement categories endpoint functions

[sboysel]

Addresses #22

[codecov-io]

Codecov Report

Merging #37 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #37      +/-   ##
=========================================
+ Coverage   99.65%   99.7%   +0.04%     
=========================================
  Files          24      30       +6     
  Lines         583     668      +85     
=========================================
+ Hits          581     666      +85     
  Misses          2       2

Impacted Files	Coverage Δ
R/fredr_category_tags.R	100% <100%> (ø)
R/fredr_category_related_tags.R	100% <100%> (ø)
R/validation.R	98.55% <100%> (+0.03%)	⬆️
R/fredr_category_related.R	100% <100%> (ø)
R/fredr_category_children.R	100% <100%> (ø)
R/fredr_category.R	100% <100%> (ø)
R/fredr_category_series.R	100% <100%> (ø)
... and 3 more

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