Access to ref and properties of the contenteditable div in the MessageInput

Sometimes we need to set the properties of the inner contenteditale div in MessageInput.
For example to set tabIndex, aria-* attributes, or get a reference to manipulate the content directly.

Now this is only possible using querySelector, but this is not the React way.

So we need to add a new property (eg. inputProps) to the **<MessageInput />** component to enable passing properties directly to the inner div.

Related: #93, #94