ncc run to use sourcemaps by default

[guybedford]

I think by default having the good debugging experience makes sense here, as the sourcemaps generation time isn't too long at all.

[codecov-io]

Codecov Report

Merging #230 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #230   +/-   ##
=====================================
  Coverage      70%    70%           
=====================================
  Files          13     13           
  Lines         670    670           
=====================================
  Hits          469    469           
  Misses        201    201

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

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