feat(cli): add basis for new "emacs-builder" CLI tool written in Go

This will eventually replace the build-emacs-for-macos script that's
written in Ruby. The migration of functionality will happen bit by bit
over time.

cmd/emacs-builder/main.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/jimeh/build-emacs-for-macos/pkg/cli"
+)
+
+var (
+	version string
+	commit  string
+	date    string
+)
+
+func main() {
+	cliInstance := cli.New(version, commit, date)
+
+	err := cliInstance.Run(os.Args)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "ERROR: %s\n", err.Error())
+		os.Exit(1)
+	}
+}