Add a release system. (#550)

* feat: add release system.
* feat: goreleaser.
* review: minor changes.

cli.go

@@ -7,7 +7,6 @@ import (
 	"log"
 	"os"
 	"path"
-	"strings"
 	"text/tabwriter"
 
 	"github.com/urfave/cli"
@@ -25,18 +24,15 @@ func logger() *log.Logger {
 	return Logger
 }
 
-var gittag string
+var (
+	version = "dev"
+)
 
 func main() {
 	app := cli.NewApp()
 	app.Name = "lego"
 	app.Usage = "Let's Encrypt client written in Go"
 
-	version := "0.4.1"
-	if strings.HasPrefix(gittag, "v") {
-		version = gittag
-	}
-
 	app.Version = version
 
 	acme.UserAgent = "lego/" + app.Version