Active and Inactive memory for mem

[goller]

I'd like to have access to active and inactive memory.

gopsutil returns it:
https://github.com/shirou/gopsutil/blob/master/mem/mem_linux.go#L46
but telegraf doesn't export it:
https://github.com/influxdata/telegraf/blob/master/plugins/inputs/system/memory.go#L27