Initial implementation of Slider Component

[vipentti]

Initial implementation of a Slider component to resolve #69.

Did a major refactor of the API based on the discussion.

The component does not currently use or support Parts component model because the component itself has no need for update because of the user having to provide both onChange and value calls to update the slider.

Slider does not currently work properly on Microsoft Edge. It also does not currently look the best on regular Internet Explorer due to flexbox. However, it is functional.

Example use:

import Material.Slider as Slider

slider : Model -> Html Msg
slider model =
    p [ style [ ("width", "300px") ] ]
    [ Slider.slider
        [ Slider.onChange SliderMsg
        , Slider.value model.value
        ]
    ]

This follows the MDL of wrapping the actual slider in a separate element that has a width specified.

[debois]

Very good—I merged this, leaving open issues #75 and #76 about IE/Edge compatibility.