modules: creates disk-io-module

This creates the disk-io-module, which displays io information read from
`/proc/diskstats`. Details on `diskstats` can be found on:
https://www.kernel.org/doc/Documentation/ABI/testing/procfs-diskstats

doc/yambar-modules-disk-io.5.scd

@@ -0,0 +1,85 @@
+yambar-modules-disk-io(5)
+
+# NAME
+disk-io - This module keeps track of the amount of bytes being
+read/written from/to disk. It can distinguish between all partitions
+currently present in the machine.
+
+# TAGS
+
+[[ *Name*
+:[ *Type*
+:[ *Description*
+|  device
+:  string
+:  Name of the device being tracked (use the command *lsblk* to see these).
+   There is a special device, "Total", that reports the total stats
+   for the machine
+|  is_disk
+:  boolean
+:  whether or not the device is a disk (e.g. sda, sdb) or a partition
+   (e.g. sda1, sda2, ...). "Total" is advertised as a disk.
+|  read_speed
+:  int
+:  bytes read, in bytes/s
+|  write_speed
+:  int
+:  bytes written, in bytes/s
+|  ios_in_progress
+:  int
+:  number of ios that are happening at the time of polling
+
+# CONFIGURATION
+
+[[ *Name*
+:[ *Type*
+:[ *Req*
+:[ *Description*
+|  interval
+:  int
+:  no
+:  Refresh interval of disk's stats in ms (default=500).
+   Cannot be less then 500 ms
+
+# EXAMPLES
+
+This reports the total amount of bytes being read and written every second,
+formatting in b/s, kb/s, mb/s, or gb/s, as appropriate.
+
+```
+bar:
+  left:
+    - disk-io:
+        interval: 1000
+        content:
+          map:
+            conditions:
+              device == Total:
+                list:
+                  items:
+                    - string: {text: "Total read: "}
+                    - map:
+                        default: {string: {text: "{read_speed} B/s"}}
+                        conditions:
+                          read_speed > 1073741824:
+                            string: {text: "{read_speed:gib} GB/s"}
+                          read_speed > 1048576:
+                            string: {text: "{read_speed:mib} MB/s"}
+                          read_speed > 1024:
+                            string: {text: "{read_speed:kib} KB/s"}
+                    - string: {text: " | "}
+                    - string: {text: "Total written: "}
+                    - map:
+                        default: {string: {text: "{write_speed} B/s"}}
+                        conditions:
+                          write_speed > 1073741824:
+                            string: {text: "{write_speed:gib} GB/s"}
+                          write_speed > 1048576:
+                            string: {text: "{write_speed:mib} MB/s"}
+                          write_speed > 1024:
+                            string: {text: "{write_speed:kib} KB/s"}
+```
+
+# SEE ALSO
+
+*yambar-modules*(5), *yambar-particles*(5), *yambar-tags*(5), *yambar-decorations*(5)