Allow Classname passtrough for data items

[masbaehr]

This would help for adding additional functionality (e.g. tooltips) or conditionally rendered data items.

{
        id: counter,
        start: new Date(),
        end: new Date(),
        name: "myItem",
        color: "#FF0000",
        class: "eventType1"
      }

This allows me to select items with class "eventType1" and do additional work on them.
An even better approach would be to pass-trough all data-properties. But i was not aware how to do that quickly..

[codecov]

Codecov Report

Merging #34 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   79.86%   79.93%   +0.06%     
==========================================
  Files          16       16              
  Lines         591      593       +2     
==========================================
+ Hits          472      474       +2     
  Misses        119      119

Impacted Files	Coverage Δ
src/lib/components/viewport/DataViewPort.js	74.07% <ø> (ø)	⬆️
src/lib/components/viewport/DataTask.js	70.58% <100%> (+0.7%)	⬆️

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