Exclude target_os=wasi from browser

[bkolobara]

Closes #219

There are many new WebAssembly runtimes intended to be run outside of the browsers. This pull request makes plotters compatible with them.

The main issue is that wasm-bindgen doesn't make sense when the target os is specified as wasi and there is no JavaScript environment. Wasi already exposes low level APIs that can be used instead.

A nice side-effect of this pull request is that you can now choose to compile with target=wasm32-wasi and still run your .wasm file in the browser, but provide a WASI polyfill instead of wasm-bindgen.

[codecov]

Codecov Report

Merging #220 (77f5823) into master (96d5184) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   68.83%   68.63%   -0.21%     
==========================================
  Files          56       56              
  Lines        4313     4301      -12     
==========================================
- Hits         2969     2952      -17     
- Misses       1344     1349       +5     

Impacted Files	Coverage Δ
src/element/image.rs	0.00% <ø> (ø)
src/style/font/mod.rs	0.00% <ø> (ø)
src/style/size.rs	75.00% <ø> (ø)
src/chart/axes3d.rs	55.78% <0.00%> (-3.16%)	⬇️
src/coord/ranged3d/projection.rs	73.86% <0.00%> (-1.14%)	⬇️
src/chart/series.rs	61.70% <0.00%> (-1.07%)	⬇️
src/chart/mesh.rs	34.78% <0.00%> (-0.97%)	⬇️
src/chart/context.rs	82.83% <0.00%> (-0.47%)	⬇️
src/coord/ranged1d/types/numeric.rs	86.08% <0.00%> (-0.12%)	⬇️
src/data/float.rs	93.75% <0.00%> (-0.10%)	⬇️
... and 4 more

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[gilescope]

Cool as. The more stuff that compiles to wasm the better.