Add support for color input

[mcintyre94]

Ref: #321

This PR adds a new input type Color. This is rendered using <input type="color"> and stores values of the form #ea1f2d

• I needed to special-case the input CSS a little bit. The padding-top + padding-bottom is removed from color inputs because it was reducing the amount of space where the color is visible to be too narrow. A width is added because without this it was matching the width of its label. I guess this is slightly odd default sizing of the HTML input itself, but I'm not super familiar with it!
• I've introduced a function html_input_type to determine the value of the <input> type attribute. For the existing :number and :url types I've set this to use the HTML number and url types respectively. Let me know if this isn't desirable and I can change any of them back to text.

This works really nicely as a reactive input with VegaLite graphs:

livebook-color-reactive.mov

[jonatanklosko]

Perfect, thanks!