Require main support

[guybedford]

Supports treating "require.main" as a passthrough value so that it is the application-level require.main in the output.

[guybedford]

Fixes #83.

[codecov-io]

Codecov Report

Merging #84 into master will increase coverage by 0.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   67.69%   68.06%   +0.36%     
==========================================
  Files           3        3              
  Lines         260      263       +3     
==========================================
+ Hits          176      179       +3     
  Misses         84       84

Impacted Files	Coverage Δ
src/asset-relocator.js	99.21% <100%> (+0.01%)	⬆️

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