Add size prefix is not present

1. Lets assume folder containing just files:

```
drop-index.png
reindex.png 
```
1. Add package with this folder
2. Generate sprite

Expected:
CSS class names should be

```
icon-16-drop-index
icon-16-reindex
```

Current:
Does not match sprite image CSS selector

```
icon-drop-index
icon-reindex
```
