Feature Request: Expose PSI Information

With v4.20 the Linux kernel introduced a new metric for system usage, similar to the system load averages, called “Pressure Stall Information“ (PSI).
More infos about it on [LWN.net](https://lwn.net/Articles/759658/) or in `Documentation/accounting/psi.txt` of the Linux sources.
I’d like to show the PSI information using i3status.

It would be necessary to check whether the kernel has `CONFIG_PSI` enabled, which means checking whether the files in `/proc/pressure` exist. Then these files would have to be parsed and the information inserted into the format string given by the user in the i3status-config.

The documentation this feature in i3status should note, that the PSI is only available if the kernel has a version of at least 4.20 and was built with `CONFIG_PSI` enabled.

Currently there are rather many (15) numbers exposed by the PSI. (3 for cpu, 6 each for memory and io)
I don’t know how the config options for the PSI should look like so that they are easy to use. Something similar to the load average is probably a good idea.