Implement unit and e2e tests

[axe312ger]

This PR adds basic unit and e2e tests to ensure we won't accidentally break functionality while implementing #30

What the pr does:

• first restructuring of the source code to improve readability and prepare for further changes
• adds unit tests for core functionality and the node api (mostly snapshots, more precise tests follow later on in other PRs)
• e2e tests for cli usage

Would be great if @technopagan could enable travis for this repo now :)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (next@84ef7d4). Click here to learn what that means.
The diff coverage is 95.52%.

@@           Coverage Diff           @@
##             next      #32   +/-   ##
=======================================
  Coverage        ?   86.53%           
=======================================
  Files           ?        5           
  Lines           ?      104           
  Branches        ?       15           
=======================================
  Hits            ?       90           
  Misses          ?       11           
  Partials        ?        3

Impacted Files	Coverage Δ
src/utils/primitive.js	100% <100%> (ø)
src/utils/svg.js	100% <100%> (ø)
src/utils/helpers.js	100% <100%> (ø)
src/index.js	71.73% <70%> (ø)

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[efegurkan]

LGTM, feel free to merge 👍