Easy way to redefine an action?

### I'm submitting a...
- [ ] Bug report
- [x] Feature request

I would like to redefine some actions. The only way I found to do this is to use the `toolbar` option, but this one overwrite all default items, so I need to set all default icons and create a new icon with the same values instead the custom action, like this:

```js
toolbar: [
'bold', 'italic', 'heading', '|', 'quote', 'unordered-list', 'ordered-list', '|', 'link',
{
name: 'image',
action: function customFunction(editor){
alert('Hey!')
},
className: 'fa fa-image',
title: 'Insert image'
},
'|', 'preview', 'side-by-side', 'fullscreen', 'guide'
]
```

Is there an easier way to do this?

If not, I would suggest a `toolbarAction` option, which could work like this:

```js
new EasyMDE({
toolbarAction: {
image: function customFunction(editor){
alert('Hey!')
},
}
});
```

Start by tracing how the toolbar option and the EasyMDE constructor process toolbar items. Determine whether a toolbarAction-style option can override one named action while preserving the default items; done means the image action can be replaced without copying the entire toolbar configuration, with existing toolbar behavior retained.

Written by the indexing model from the issue text.