Add optional parameter specifying an application name

Thank you very much for `node-open`!

I use it to open a browser window to run my library's integration tests.

Everything works perfectly, until I want to use a non-default browser to run my tests. I did a bit of research, and added support for specifying an optional application name in `open`, like this:

```javascript
open('http://google.com', 'firefox');
```

I hope you will consider merging this patch! Thank you for making my life easier with `node-open`!