Mac OSX support

[kleinjm]

This is a feature request to support Mac OSX. There is no directory and no network interfaces list at /sys/class/net/. I'm not sure what the best tool for Mac would be.

Perhaps display a warning if no network interfaces are found here as well.

Thanks in advance for the consideration, it would be a very useful tool.

[ChaserTlss]

https://github.com/minhdanh/tmux-network-speed/tree/master
This is I find a tmux plugin can work in macOS.

[taizilongxu]

https://github.com/xamut/tmux-network-bandwidth
This works too