Add loglevel option

At some point, I won't need the logs in the console or the output file.

Take a look at the [tun2socks](https://github.com/xjasonlyu/tun2socks/blob/4127937ea7c450a5230b273f406c9410acec2be7/main.go#L33) implementation.
They used [Uber's zap](https://github.com/uber-go/zap) package.
```
[debug|info|warn|error|silent]
```