Externals mapping support

[guybedford]

This adds support for an object form in externals to support remapping the externals value in the target environment.

This is a necessary feature for the next.js support.

[codecov-io]

Codecov Report

Merging #402 into master will increase coverage by 1.19%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
+ Coverage   73.29%   74.49%   +1.19%     
==========================================
  Files          14       14              
  Lines         397      396       -1     
==========================================
+ Hits          291      295       +4     
+ Misses        106      101       -5

Impacted Files	Coverage Δ
src/@@notfound.js	0% <0%> (ø)	⬆️
src/index.js	83.53% <100%> (+0.41%)	⬆️

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