Configure using appSettings

Usage:

```xml
<appSettings>
    <add key="sa.Twitter" value="key:Rb7qNNPUPsRSYkznFTbF6Q;secret:pP1jBdYOlmCzo08QFJjGIHY4YSyPdGLPO2m1q47hu9c"/>
    <add key="sa.WindowsLive" value="key:00000000400ED488;secret:OAc-A5hoXE0eOolc6aczF2xvnq5sLfRr"/>
    <add key="sa.GitHub" value="key:9403c7920a82689969d1;secret:e5b3807c7c97466634bdf21ddf9a179485f1fe60"/>
</appSettings>
```

Value is:

`key:*key*;secret:*secret*;scopes:*scopes*`

Scopes is optional.