Add a shell executable

```sh
$ npm install -g fastlog
$ fastlog debug 'this will not show'
$ FASTLOG_LEVEL=debug fastlog debug 'this will show'
[Sun, 30 Nov 2014 23:58:07 GMT] [debug] [default] this will show
$ FASTLOG_CATEGORY=security fastlog info 'watch out'
[Sun, 30 Nov 2014 23:58:49 GMT] [info] [security] watch out
```