Support -e flag in ncc run

[guybedford]

This works by creating a symlink to the local node_modules folder in the temporary build location.

[guybedford]

Fixes #154.

[codecov-io]

Codecov Report

Merging #218 into master will decrease coverage by 1.13%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
- Coverage   68.84%   67.71%   -1.14%     
==========================================
  Files          13       13              
  Lines         597      607      +10     
==========================================
  Hits          411      411              
- Misses        186      196      +10

Impacted Files	Coverage Δ
src/cli.js	0% <0%> (ø)	⬆️

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