Reconsider use of icon in Banner component

[jes708]

Allow icon props, such as size, to be configurable.
Remove presets named after colors, since different palettes will not translate the same
Remove the showIcon prop since there will no longer be presets

https://github.com/priceline/design-system/blob/master/packages/core/src/Banner.js#L89

[craigpalermo]

Ideas discussed in WG:

• Might accept Icon as a prop (but this would be a breaking change)
  • Could also be done as an override prop
• In the meantime, we could just change the color names