Is your feature request related to a problem? Please describe.

I'm unable to easily show my mac address in the internal/network module type

Why does polybar need this feature?

It will allow people to easily see their current mac address without the need for a custom script.

Describe the solution you'd like

There already exist things like %ifname% and %local_ip% which show information about the current network. How about adding %mac_addr%

Describe alternatives you've considered

Creating a custom script in order to read the output of /sys/class/net/<interface>/address however this solution doesn't allow me to put my mac address in my network module's text

Additional context

In order to get the mac address of a certain interface you can read the directory /sys/class/net/<interface>/address