Add support for mocking config calls

[flimzy]

Relates to go-kivik/kivik#390

[codecov]

Codecov Report

Merging #55 into master will increase coverage by 1.22%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   83.35%   84.58%   +1.22%     
==========================================
  Files          19       19              
  Lines        1328     1408      +80     
==========================================
+ Hits         1107     1191      +84     
+ Misses        201      197       -4     
  Partials       20       20

Impacted Files	Coverage Δ
gen/parse.go	88% <ø> (ø)	⬆️
client.go	100% <ø> (ø)	⬆️
expectations.go	96.94% <100%> (+1.51%)	⬆️
dbexpectations.go	98.78% <100%> (-0.01%)	⬇️
util.go	100% <100%> (ø)	⬆️
gen/main.go	29.57% <20%> (ø)	⬆️
gen/render.go	56.22% <33.33%> (+0.88%)	⬆️

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