New feature: File upload

[ergisgjergji]

• Added the ability to be able to select file(s) aside text. Now, the signature of onSendMessage(message) is onSendMessage(message, files = []).
• Added a prop for message button's click handling: onMessageButtonClick
• Removed onSendKey as obsolette. The default behaviour now is: enter to send message / shift+enter inserts a new line in the textArea

To view the full details, I have published my fork in npm, under: @ergisgjergji/react-chat-plugin (I am personally using it in production).
You can view the updated screenshot, the changelog, examples and the props table.