Ch/animate text and graphics

[ccchwang]

• Adds a brief fade-in to text components (main heading, subheadings, description, button) when scrolling through sections so they don't change abruptly
• Fades out graphics when they start moving out of viewport
• Fades in graphics when they start coming into viewport

[codecov-io]

Codecov Report

Merging #471 into master will decrease coverage by 0.75%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
- Coverage   93.23%   92.48%   -0.76%     
==========================================
  Files          30       30              
  Lines        1065     1065              
  Branches      199      199              
==========================================
- Hits          993      985       -8     
- Misses         61       64       +3     
- Partials       11       16       +5

Impacted Files	Coverage Δ
packages/microcosm/src/history.js	78.03% <0%> (-9.85%)	:arrow_down:
packages/microcosm/src/action.js	89.74% <0%> (-9.41%)	:arrow_down:
packages/microcosm/src/domain-engine.js	80.88% <0%> (+1.47%)	:arrow_up:
packages/microcosm/src/microcosm.js	95.28% <0%> (+4.71%)	:arrow_up:
packages/microcosm/src/utils.js	98.63% <0%> (+5.47%)	:arrow_up:
packages/microcosm/src/lifecycle.js	100% <0%> (+33.33%)	:arrow_up:
packages/microcosm/src/install-devtools.js	100% <0%> (+33.33%)	:arrow_up:

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