improve svg sprites

[cyrilwanner]

• Add example for SSR to the readme
• Also improve svg icons used in sprites
• Use a runtime generator for easier usage

[codecov]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #13   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          42     43    +1     
  Branches       13     13           
=====================================
+ Hits           42     43    +1

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️

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