rewrite html generator with html/template

internal/disk_usage/disk_usage.go

@@ -0,0 +1,21 @@
+package disk_usage
+
+import "errors"
+
+var ErrNotSupported = errors.New("disk_usage: not supported on this OS")
+
+// Used + Free can be less than Total: space reserved for root/admin
+// is not free for this process, but not used either.
+type Usage struct {
+	// size of the file system (available to this user, if there are quotas)
+	Total int64
+	// Total minus all free space, including reserved
+	Used int64
+	// available to this process
+	Free int64
+}
+
+// DiskUsage returns usage of the file system that contains path.
+func DiskUsage(path string) (Usage, error) {
+	return diskUsage(path)
+}