version: Add a version command

Fixes: #32

Makefile

@@ -2,8 +2,10 @@
 
 all: jk
 
+VERSION := $(shell git describe)
+
 jk: pkg/__std/lib/assets_vfsdata.go
-	GO111MODULE=on go build -o $@
+	GO111MODULE=on go build -o $@ -ldflags "-X main.Version=$(VERSION)"
 
 pkg/__std/lib/assets_vfsdata.go: std/build/std.js
 	GO111MODULE=on go generate ./pkg/__std/lib

version.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+var versionCmd = &cobra.Command{
+	Use:   "version",
+	Short: "Print jk's version",
+	Run:   version,
+}
+
+func init() {
+	jk.AddCommand(versionCmd)
+}
+
+// Version is jk's version. It's populated a link time.
+var Version = "git"
+
+func version(cmd *cobra.Command, args []string) {
+	fmt.Println("version:", Version)
+}