Added feature to customize server address in console

Fix for issue [#466](https://github.com/tommy351/hexo/issues/466). 

Able to override the default server address in console when you run `hexo server`.

Server address can be set either in `_config.yaml` or run with `-a` or `--address` options. Default is localhost. Note that the server is still running in localhost/127.0.0.1 obviously.