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>

go.mod

@@ -3,6 +3,8 @@ module git.laidback.moe/shinyoukai/mikuru
 go 1.24.0
 
 require (
+	github.com/sirupsen/logrus v1.9.3
+	github.com/spf13/cobra v1.10.2
 	github.com/tj/go-editor v1.0.0
 	go.yarn.social/client v0.0.0-20250420114029-410ad71a453e
 	golang.org/x/term v0.38.0
@@ -10,8 +12,9 @@ require (
 )
 
 require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/pkg/errors v0.8.1 // indirect
-	github.com/sirupsen/logrus v1.9.3 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
 	github.com/stretchr/testify v1.11.1 // indirect
 	go.yarn.social/types v0.0.0-20250420113154-c5a6df6d2f22 // indirect
 	golang.org/x/sys v0.39.0 // indirect