Sourcemaps support

[guybedford]

This adds a --source-map / -s flag for generating source maps through the CLI.

In addition, when used with ncc run, source maps support is automatically injected into Node with a build of source-map-support/register.

In the API I changed the input option from sourceMap: true to sourcemap: true as it seemed more consistent. Happy to change this back if anyone disagrees. Also added a filename option to the API as the source map needs a filename to reference to.

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 0.51%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   67.69%   67.17%   -0.52%     
==========================================
  Files           3        3              
  Lines         260      262       +2     
==========================================
  Hits          176      176              
- Misses         84       86       +2

Impacted Files	Coverage Δ
src/cli.js	0% <0%> (ø)	⬆️
src/index.js	91.37% <100%> (ø)	⬆️

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[rauchg]

Question: how expensive are source maps? In other words, is there a case for not enabling them for the prod output?

[guybedford]

Hmm, I just tested the average time it takes the tests to run with vs without sourcemaps and it was 67s with and 61s without.

I guess I'm just used to builds where source maps times dominate build times... but that certainly isn't the case here.

I've added a commit to set the default to use sourcemaps.

We could still stick with having them off for build by default, although that would need a separate --source-map flag.