fix: -V flag (#14)

.goreleaser.yml

@@ -4,6 +4,8 @@ builds:
       - CGO_ENABLED=0
     flags:
       - -trimpath
+    ldflags:
+      - -s -w -X main.version={{.Version}}
     targets:
       - darwin_amd64
       - darwin_arm64

cmd/musttag/main.go

@@ -1,11 +1,30 @@
 package main
 
 import (
-	"golang.org/x/tools/go/analysis/singlechecker"
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
 
 	"github.com/junk1tm/musttag"
+	"golang.org/x/tools/go/analysis/singlechecker"
 )
 
+var version = "dev" // injected at build time.
+
 func main() {
+	// override the builtin -V flag.
+	flag.Var(versionFlag{}, "V", "print version and exit")
 	singlechecker.Main(musttag.New())
 }
+
+type versionFlag struct{}
+
+func (versionFlag) IsBoolFlag() bool { return true }
+func (versionFlag) Get() interface{} { return nil }
+func (versionFlag) String() string   { return "" }
+func (versionFlag) Set(string) error {
+	fmt.Printf("musttag version %s %s/%s\n", version, runtime.GOOS, runtime.GOARCH)
+	os.Exit(0)
+	return nil
+}