Improve disk code to return back disk StatFS{} structure

```
StatFS {
Total int64
Free int64
FSType string
}
```

Provides more information in a cross platform way.

pkg/disk/disk.go

@@ -0,0 +1,11 @@
+package disk
+
+// StatFS stat fs struct is container which holds following values
+// Total - total size of the volume / disk
+// Free - free size of the volume / disk
+// FSType - file system type string
+type StatFS struct {
+	Total  int64
+	Free   int64
+	FSType string
+}