Switch to Cobra

Also:
- Fixed the Makefile
- Added a Taskfile.yml (see: https://taskfile.dev)
- Updated the manual page

Signed-off-by: Shin'ya Minazuki <shinyoukai@laidback.moe>

cmd/main.go

@@ -0,0 +1,5 @@
+package main
+
+func main() {
+	Execute()
+}