support for message deletion

[mattmezza]

It would be nice if react-beautiful-chat could support message deletion, here below I describe how to implement it (probably not the best way to do it: beware!):

1. pass a function msg => { /*customCodeForDeletion()*/ } to the onDelete props in the Launcher component
2. if the props is not null, the message components should support deletion by showing an appropriate UI element with an onClick handle that fires the passed in props passing as a parameter the msg object

By implementing this solution we would leave to the RBC user the freedom to choose whichever ID they wanna use for messages identification: they can choose to set an id property on the message object that will be given back to the onDelete function to use for the api call.