add envsubst cli command

.gitignore

@@ -1 +1,2 @@
+/envsubst
 coverage.out

cmd/envsubst/main.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"bufio"
+	"os"
+	"log"
+	"fmt"
+
+	"github.com/drone/envsubst"
+)
+
+func main() {
+	stdin := bufio.NewScanner(os.Stdin)
+	stdout := bufio.NewWriter(os.Stdout)
+
+	for stdin.Scan() {
+		line, err := envsubst.EvalEnv(stdin.Text())
+		if err != nil {
+			log.Fatalf("Error while envsubst: %v", err)
+		}
+		_, err = fmt.Fprintln(stdout, line)
+		if err != nil {
+			log.Fatalf("Error while writing to stdout: %v", err)
+		}
+		stdout.Flush()
+	}
+}
+