Add optional onToggleFullScreen support from options

[n-3-0]

I have a project that wraps your editor in a React component, and I needed a way to append a class to a wrapper component. This change allowed me to set my component's state, thereby giving me the opportunity to conditionally style it.
Here is how I use your library, as well as the proposed change:

import EasyMDE from "easymde/src/js/easymde";

//In the component class
componentDidMount() {
    this.mde = new EasyMDE({
        onToggleFullScreen: this.onToggleFullscreen,
        //...the rest of the properties I pass in
    });
    //... the rest of my code here uses the "blur" and "change" events of this.mde.codemirror to bridge the React gap
}

//A single parameter is passed in, which is the current fullscreen state of the editor
onToggleFullscreen = (fullscreen) => {
    this.setState({ fullscreen });
}

This change works with both the side-by-side mode, and the fullscreen mode, as well as any call to EasyMDE.toggleFullScreen or EasyMDE.prototype.toggleFullScreen that other developers might use. I've thoroughly tested it with my project, and so far I've been unable to break it unless someone enters a non-function truthy value for onToggleFullScreen.

I'm not sure how to create a JSFiddle for this specific project, but since it's a small change I'm not sure if it's entirely warranted. Finally, I'd be happy to add the necessary documentation for this, but I'm not sure if I should add a section to the EasyMDE part of the README, or append it to the SimpleMDE part that has the rest of the props.

[Ionaru]

I'll add the documentation to the SimpleMDE part for consistency, but in the future that'll get rewritten.