Add method to allow the adding of custom bindings

[sambenne]

I wanted to add a new keyboard shortcut but was unable to. This is because the list of bindings was set and no way to update it. This change adds a method that allows the user to add new bindings so you can have custom keyboard shortcuts

[sambenne]

So for instance you can then do:

EasyMDE.prototype.addBinding('submitModal', keyboardAction);

Then when you are creating the instance you can do:

shortcuts: {
    "submitModal": "Ctrl-Enter"
}

[Ionaru]

This looks great! I'm gonna do some testing and add the typings soon!