Add flow

[yavorsky]

Also replaced uglify with useSyntax like in #264 and will pull changes after use-syntax-option into 2.0 merge.

[codecov-io]

Codecov Report

Merging #269 into 2.0 will decrease coverage by 0.35%.
The diff coverage is 98.52%.

@@            Coverage Diff             @@
##              2.0     #269      +/-   ##
==========================================
- Coverage   96.06%   95.71%   -0.36%     
==========================================
  Files           9        9              
  Lines         407      420      +13     
  Branches      129      128       -1     
==========================================
+ Hits          391      402      +11     
- Misses          4        5       +1     
- Partials       12       13       +1

Impacted Files	Coverage Δ
src/targets-parser.js	94.23% <100%> (-2.14%)	⬇️
src/use-built-ins-entry-plugin.js	100% <100%> (ø)	⬆️
src/utils.js	100% <100%> (ø)	⬆️
src/normalize-options.js	100% <100%> (ø)	⬆️
src/index.js	97.67% <100%> (+0.08%)	⬆️
src/use-built-ins-plugin.js	93.7% <87.5%> (-0.75%)	⬇️

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[existentialism]

@yavorsky mind rebasing?

[yavorsky]

@existentialism yeah, on it

[existentialism]

@yavorsky hope you don't mind, wanted to get this landed so I resolved the conflicts.

Small tweaks:

• Bumped flow to 0.45
• Reverted renaming modules option to moduleType (we can do that separately, as we may want to discuss whether or not we want that breaking change, whether to preserve backwards compatibility, and consistency with other presets, etc)

Otherwise, your changes look awesome, mind giving this a review?

[yavorsky]

@existentialism Oh, thanks!
Yeah. I think it's ready to be merged. 🕺
Also, we could add scripts/test/*.js files in the future.