mods to core/config.clj and added test for it

[gkspranger]

• using idiomatic if-not
• added arity/1 to prefixed-env-vars to allow for user defined env vars, mainly for testing .. arity/0 (original func signature) passes in environ.core/env to arity/1
• added arity/1 and arity/2 to config-from-env-or-file to allow for user defined file cfgs and env cfgs .. arity/0 (original func signature) passes in load-edn! and prefixed-env-vars to arity/2 -- which were previously in the body of the function
• added many (comment)
• added test for core/config.clj

not sure how you are going to feel about me modifying the arity of those functions .. it won't hurt my feelings if you want to reject, i am just trying to find a "pure" way i can test these functions that don't rely on environmental factors ..

[codecov[bot]]

Codecov Report

Merging #163 (c24ebd4) into master (9f6f7b6) will increase coverage by 0.33%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
+ Coverage   46.44%   46.77%   +0.33%     
==========================================
  Files          94       94              
  Lines        4119     4126       +7     
  Branches      197      197              
==========================================
+ Hits         1913     1930      +17     
+ Misses       2009     1999      -10     
  Partials      197      197              

Impacted Files	Coverage Δ
src/yetibot/core/config.clj	78.12% <73.33%> (+43.50%)	:arrow_up:
src/yetibot/core/util/config.clj	56.25% <100.00%> (+1.41%)	:arrow_up:

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