Separate minify compress option

[guybedford]

This adds a separate compress / --compress CLI and API option, that need to be explicitly opted into for terser compression.

The size of the ncc build itself is exactly the same with and without Terser compression, yet takes 27s with compression, vs 25.7 without compression.

[guybedford]

Given the lack of any effect of terser compression, it could even be worth disabling it completely and not even having a flag here.

I've added another commit that does that, which can be reverted if this is really needed, but I don't see a good reason to include it personally since it seems Webpack is already doing the most basic optimizations for us anyway.

[guybedford]

(When I mean that the build size is exactly the same, I really do mean it is identically 5,150,515 bytes between the two)

Correction: verified this again and got 5,150,480 bytes for the compressed build.

[codecov-io]

Codecov Report

Merging #161 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #161   +/-   ##
======================================
  Coverage    72.3%   72.3%           
======================================
  Files          10      10           
  Lines         473     473           
======================================
  Hits          342     342           
  Misses        131     131

Impacted Files	Coverage Δ
src/index.js	93.15% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f7bc7a0...9a11a4e. Read the comment docs.