join powerlin_args

fix problem shown below:

```
==> fontforge -lang=py -script /private/tmp/ricty-1xeZ/Ricty-3.2.3/scripts/powerline-fontpatcher [] Ricty-Bold.ttf
 Executable based on sources from 14:57 GMT 31-Jul-2012-D.
 Library based on sources from 14:57 GMT 31-Jul-2012.
usage: powerline-fontpatcher [-h] [--no-rename] [--source-font font]
                             font [font ...]
powerline-fontpatcher: error: argument font: can't open '[]': [Errno 2] No such file or directory: '[]'
```