text: Title: Marks in Range Slider has height equal to 0
  Body: And, therefore, are hidden.
  
  I have a range slider followed by an array of plotly graphs. I've set some marks for the slider but they aren't visible. Apparently, they are inside a div with class "rc-slider-mark" and that div has 0 height and is absolute positioned, so `overflow: overlay` doesn't help either. This is what I see:
  ![screenshot from 2017-12-13 01-35-07](https://user-images.githubusercontent.com/2631430/33915988-d53764e2-dfa6-11e7-92f2-e661e1ef8aa9.png)
  
  I worked around this by setting a fixed height in a div that wraps the RangeSlider component. The height had to be bigger enough to contain the slider plus the marks that were not being shown, this is what I get:
  ![screenshot from 2017-12-13 01-35-57](https://user-images.githubusercontent.com/2631430/33916059-33b25400-dfa7-11e7-8905-f6e4b8977926.png)
  
  However, messing up with fixed heights and absolute positions doesn't look very good to me. I think there should be a better css implementation that maintains the needed height for the marks, while at the same time displays the marks exactly wherever they have to be.
  
  The whole live example is available [here](http://wikichron.science/).