Add `ps_disk_io_counters()` for Windows and Linux

[michaelwalshe]

Implement ps_disk_io_counters for Linux and Windows based on psutil.

Adds new functions

• ps_disk_io_counters: Exported user-facing function, to return data.frame of disk IO counters. Takes 1 argument, perdisk, whether to return a total or 1 row per disk
• ps__disk_io_counters_linux: Internal function for linux implementation
• ps__disk_io_counters_windows: Internal function for windows implementation
• ps__is_storage_device: Whether a device is a root or logical partition (linux)
• ps__read_procfs: Read disk stats from /proc/diskstats (linux)
• ps__read_sysfs: Fallback for if /proc/diskstats doesn't exist, reads from /sys/block (linux)

Plus the following C function for windows:

• ps__disk_io_counters: Internal function to return disk IO counters per disk

Implementation of all functions heavily based on implementation from https://github.com/giampaolo/psutil/blob/master/psutil/_pslinux.py

User facing ps_disk_io_counters is tested, although not extensively as that requires mocking e.g. readLines which testthat currently cannot do.

Closes #145.

[michaelwalshe]

Hi @gaborcsardi, have just updated the NEWS.md so this should be ready for review/merge now, please let me know if there's anything I need to change or update.

[gaborcsardi]

Thank you and sorry for the long wait! I adjusted it a bit, most importantly I removed the perdisk parameter, and added a macOS implementation. Thanks again!