FontAwesome 5 compatibility

[Ionaru]

Added compatibility for FontAwesome 5 by creating a seperate element for the icon and nesting it inside the button element instead of putting the classes on the button element. This makes sure that the button and all its properties stays intact when FontAwesome 5 changes elements to <svg> for their icons.

FontAwesome 4 works with this change as well. There are no visual differences except that the icon has shifted 1px up, probably because of some centering by FontAwesome.

Fixes #664

[johndodev]

Need plz !

[Ionaru]

This has been implemented in my fork of the project: https://github.com/Ionaru/easy-markdown-editor

[Jetski5822]

Any chance this could be added in?

[atodorov]

Is there anyone who wants to co-maintain this package? I've pinged the original authors to see if they need help but my expertise is in other languages so would appreciate backup.