Improve API for getSimulator

[AlexMSmithCA]

Consistency with getService (link)

[codecov]

Codecov Report

Merging #120 into master will increase coverage by 1.7%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #120     +/-   ##
=========================================
+ Coverage   87.69%   89.39%   +1.7%     
=========================================
  Files           3        3             
  Lines          65       66      +1     
  Branches        8        8             
=========================================
+ Hits           57       59      +2     
+ Misses          6        5      -1     
  Partials        2        2

Impacted Files	Coverage Δ
src/index.js	75.86% <100%> (+0.86%)	⬆️
src/mock-context.js	100% <0%> (+4.34%)	⬆️

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