HTML Element Placeholders

[alyssaBiasi]

In order to render custom icons as a part of the Select placeholder, we need to be able to provide styled HTML elements.

This PR:

• Adds element to the expected placeholder prop types
• Adds node to the expected placeholder prop type for Async
• Adds a smiley face icon to the CustomComponents example in the docs

I wasn't sure if it's better to add another prop (placeholderComponent or <insert better name>) in order to keep the placeholder prop a consistent type. Thoughts?

[JedWatson]

I should have known this was going to happen. Sorry @alyssaBiasi, there are conflicts now that #655 is in - would you mind updating your branch please and I'll merge this? Thanks!

[alyssaBiasi]

Ha no worries @JedWatson. I've rebased the branch and amended the commit. Now this PR just adds the node prop type to the Async component and the smiley face example 😄

[JedWatson]

Neat, thanks @alyssaBiasi!