Custom thumb's component with additional props

[BartoszKlonowski]

This pull request closes #154
It implements additional possibilities of customizing the Slider's thumb component.
Including:

• minimum and maximum value are known at level of the thumb custom component,
• as well as the currentValue so that the main, custom component can know it's current position.
• It also adds the index of where that custom step component actually is and compare it to currentValue to know whether it is pass or before the thumb ("filled" or "empty").

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What is still to be done before merging:

• document all the additional props
  Although it can be done in separate issue, as the docs would already use some attention to improve readability, especially considering extended options of customization.
• Optimize
  We currently render a lot each go, so it would be good to take a closer look
• Analyze and fix the example case
  The example implemented for new addition behaves incorrectly on iOS (the left section of custom thumb changes the position each step. It blinks for a moment before correctly render).

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Below is the short demo with the implemented example:
(iOS to the left, Android on the right)

154-custom-component.mp4