Provide flow types + a little housekeeping

[tptee]

• Update flow + flow-typed + definitions
• Update Prettier and add a config file so formatting won't go back and forth between editors
• Update flow and fix resulting errors
• Vendored out the immutable definitions to circumvent a libdef issue
• Add flow-copy-source to lib and es builds to provide Flow types

[codecov[bot]]

Codecov Report

Merging #257 into master will decrease coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   99.43%   99.43%   -0.01%     
==========================================
  Files          28       28              
  Lines         357      354       -3     
==========================================
- Hits          355      352       -3     
  Misses          2        2

Impacted Files	Coverage Δ
src/immutable/components/props-to-js.js	100% <100%> (ø)	:arrow_up:
src/reducer.js	100% <100%> (ø)	:arrow_up:
src/enhancer.js	100% <100%> (ø)	:arrow_up:
src/components/fragment.js	97.82% <100%> (ø)	:arrow_up:
src/util/normalize-href.js	100% <100%> (ø)	:arrow_up:
src/components/link.js	100% <100%> (ø)	:arrow_up:
src/immutable/components/fragment.js	100% <100%> (ø)	:arrow_up:

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