Not allowing empty config

Before:

```
[slackcat] % ./bin/slackcat wtf
2014/05/12 18:07:58 Post failed: Post : unsupported protocol scheme ""
```

After:

```
[slackcat] % ./bin/slackcat wtf
2014/05/12 18:08:05 Could not read config: Config file not found
```

Much more clear if you ask me :)