Use spf13/viper to manage config loading

[caitlinelfring]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behavior? (You can also link to an open issue here)
Manual loading of configs via gopkg.in/yaml.v2

What is the new behavior (if this is a feature change)?
Switch to using https://github.com/spf13/viper to load configs. This opens up options of other file types in the future for defining configs. Also has a lot more really awesome features that work nicely with cobra

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?)
No

Other information:

[codecov]

Codecov Report

Merging #39 (3c99caa) into main (cb8dc31) will increase coverage by 0.43%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   95.44%   95.87%   +0.43%     
==========================================
  Files          19       19              
  Lines         461      461              
==========================================
+ Hits          440      442       +2     
+ Misses         12       11       -1     
+ Partials        9        8       -1     

Impacted Files	Coverage Δ
cmd/root.go	84.21% <90.00%> (+8.12%)	⬆️
pkg/config/config.go	100.00% <100.00%> (ø)

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