Support button labels

When the medium-editor setting buttonLabels is set to something other than 'fontawesome' and provides overrides for the labels of each of the medium-editor-insert plugins, honour these overrides in the same way medium does.

This let's you do things like this, which uses Icomoon rather than font-awesome for the labels
```javascript
     var editor = new MediumEditor(editor_el, {
					buttonLabels:  {
						'img': '<i class="icon-image"></i>',
						'embed': '<i class="icon-film"></i>',
						'map': '<i class="icon-map"></i>',
						'table': '<i class="icon-table"></i>',
					}
				});
```

But in practice, any change you want to make (eg using the word 'Image' rather than 'Img') is possible.

One possible change I'm contemplating, just as I write this -  and will make if you agree: currently we use a lowercase version of the text version of the icon that you currently get (eg currently the text for images is 'Img'), but on second thought the name of the plugin (eg 'images' or 'embeds') might be better?