Add optional renderTray render prop to Editor

[blwsk]

Changes made:

• Add optional renderTray render prop to Editor component
• Add /examples/tray.html outlining the new feature
• Some "prettier" changes

Rationale:

We've built a fairly complex "message composer" component on top of draft-extend. The controls section of the Editor component contains a toolbar with many plugin buttons. Given the design we're trying to achieve, we need to access the space between the main input and the controls area in order to display validation messages and "stage" yet-to-be-published file attachment and images.

[benbriggs]

👍 thank you!