Beautify stdin if there are no filename arguments

I don't expect you to accept this pull request since I haven't signed and do not want to sign your contribution agreement thing. So this is just here in case anyone is looking for this feature, in which case they can use my fork.

This allows cssbeautify to be used as a filter, for instance calling it from vim over a group of lines, or piping curl's output to cssbeautify, and so on.

```
git clone https://github.com/tremby/cssbeautify.git
cd cssbeautify
sudo npm install -g
```