Order: mergePaths should run before collapseGroups

An SVG like this one:

```svg
<g>
    <path fill="#AC162C" d="pathdata"/>
    <path fill="#AC162C" d="pathdata-2"/>
</g>
```

…will be compressed into:

```svg
<g fill="#AC162C">
    <path d="pathdata1+2"/>
</g>
```

… which, if svgoed again, will be optimized further:

```svg
<path d="pathdata1+2" fill="#AC162C"/>
```

This is due to `collapseGroups` running before `mergePaths`, therefore not catching the newly-unnecessary `<g fill="#AC162C">`

Test SVG file: http://cl.ly/Yw2Y