add option to select type of marker

```javascript
icon = L.AwesomeMarkers.icon({
  icon: 'circle',
  prefix: 'fa',
  markerType: 'matte', // defaults to 'soft'
  markerColor: 'orange'
})

results in:

<div class="awesome-marker-icon-orange awesome-marker awesome-marker-matte ...">
  <i class="fa fa-circle  icon-white"></i>
</div>