--size-adjust for icons that appear to be cropped

Hi there, thanks for this script, that's exactly what I needed.

On my system though, some icons appear to be cropped as if their rendered size exceeded the image's. I've added a command line option that subtracts some pixels from the font size, to compensate for that.

The two PNGs below have been respectively created with:

```
./font-awesome-to-png.py --size 40 warning-sign
./font-awesome-to-png.py --size 40 --size-adjust 4 warning-sign
```

![](http://i.imgur.com/hhPltGZ.png)

__
Giuliano