Switch to using https://github.com/spf13/cobra for CLI.

Use opportunity to structure project by subcommands.

main.go

@@ -0,0 +1,15 @@
+// See cmd package.
+package main
+
+import (
+	"github.com/zrepl/zrepl/cmd"
+	"log"
+	"os"
+)
+
+func main() {
+	if err := cmd.RootCmd.Execute(); err != nil {
+		log.Printf("error executing root command: %s", err)
+		os.Exit(1)
+	}
+}