feat: support top-level width and height

[kristw]

<Vega width={xxx} height={xxx} spec={xxx} />

The top-level width and height now should work on initialization and also will have precedent over width and height in spec.

Fix #79
Replace #143

[codecov]

Codecov Report

Merging #144 into master will increase coverage by 3.72%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   32.63%   36.36%   +3.72%     
==========================================
  Files          14       15       +1     
  Lines         144      154      +10     
  Branches       37       40       +3     
==========================================
+ Hits           47       56       +9     
- Misses         89       90       +1     
  Partials        8        8

Impacted Files	Coverage Δ
...ckages/react-vega/src/utils/getUniqueFieldNames.ts	100% <ø> (ø)	⬆️
...s/react-vega/src/utils/combineSpecWithDimension.ts	100% <100%> (ø)
packages/react-vega/src/VegaEmbed.tsx	21.53% <33.33%> (+0.9%)	⬆️

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