Show zram usage

[cool-RR]

Checklist

• I've looked through the documentation and existing open issues for similar feature requests.

Describe the feature request

when showing RAM usage, would be nice to see ZRAM usage as well (compressed RAM)

[ClementTsang]

Hmm... I need to see how doable this is. Currently it would probably show under swap if that's what it's being used for. That said, it would only show basic stuff like usage, and would be grouped together with any other swapfiles/swap partitions.