WIP: slider supports dates

[ncullen93]

This is one easy way to support dates in the slider_input´ - by just calculating out custom_ticks` (ref: #227).

Still lots of work to be done here, but it's a suggestion and could be made to match the shiny functionality.

Example usage:

  slider_input("mySlider",
               min = as.Date("2019-01-23"),
               max = as.Date("2019-01-26"),
               value = as.Date("2019-01-25"))

DoD

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