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

[supersnager]

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

[aledeloss]

It would be great to also be able to add them to other elements, such as the send button (i.e. aria label)