small refactor of core/models/default-command and added tests for it

[gkspranger]

• small mod to (configured-default-command) -- using (get) vs (or), 98% sure it did not change behavior
• added/modified doc strings for some functions
• added arity/2 to some functions to allow for unit testing with custom configs
• original function signature remains, which does (get-config) and passes into arity/2
• added (comment) to exercise some code
• added midje tests
• made clj-kondo happy

a note about (fallback-enabled?) .. i didn't want to change the behavior of the function -- but it feels a bit "off", but nothing that has to be immediately changed .. let me explain ::

• (= "false" "false") => true
• (= "false" false) => false << that said, it would never make it that far because (blank? false) => true
• if true is passed into function, it blows up because (blank? true) throws runtime exception
• if "true" is passed into function, runtime is OK but obviously (= "false" "true") => false

finally, codeclimate is complaining about non-idiotmatic (not (= "false" value)) .. using (not=) fixed it and passed all tests ..

all that said -- this PR is OK for what it is supposed to do -- add tests and small refactor wins where possible ..

[codecov[bot]]

Codecov Report

Merging #170 (6fa4584) into master (2e57e69) will increase coverage by 0.09%. The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   49.54%   49.63%   +0.09%     
==========================================
  Files          94       94              
  Lines        4142     4148       +6     
  Branches      197      197              
==========================================
+ Hits         2052     2059       +7     
+ Misses       1893     1892       -1     
  Partials      197      197              

Impacted Files	Coverage Δ
src/yetibot/core/models/default_command.clj	70.00% <57.14%> (+20.00%)	:arrow_up:

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