Support both new and legacy Context APIs

[michalochman]

This PR updates <Stage /> component to work for both new and legacy Context APIs.

TODO:

• fix unit tests
• add tests for context provider

Fixes #71

[codecov]

Codecov Report

Merging #72 into master will decrease coverage by 4.36%.
The diff coverage is 67.74%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage     100%   95.63%   -4.37%     
==========================================
  Files          10       11       +1     
  Lines         206      229      +23     
==========================================
+ Hits          206      219      +13     
- Misses          0       10      +10

Impacted Files	Coverage Δ
src/index.js	100% <ø> (ø)	⬆️
src/render.js	100% <100%> (ø)	⬆️
src/Stage.js	100% <100%> (ø)	⬆️
src/AppProvider.js	60% <60%> (ø)

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