gox でビルドするバイナリの出力先を好みに合わせて変える

gox は Vagrant はじめ数々の HashiCorp プロダクトでおなじみの Mitchell Hashimoto さんによる、Go のクロスコンパイルを簡単に行うためのコマンドラインツールです。

gox でビルドを実行すると、デフォルトではファイル名に OS 名や CPU アーキテクチャ名が含まれるものになってしまいます。

```
$ gox
Number of parallel builds: 4

-->      darwin/386: github.com/yuya-takeyama/gox-test
-->    darwin/amd64: github.com/yuya-takeyama/gox-test
-->       linux/386: github.com/yuya-takeyama/gox-test
-->     linux/amd64: github.com/yuya-takeyama/gox-test
-->       linux/arm: github.com/yuya-takeyama/gox-test
-->     freebsd/386: github.com/yuya-takeyama/gox-test
-->   freebsd/amd64: github.com/yuya-takeyama/gox-test
-->     openbsd/386: github.com/yuya-takeyama/gox-test
-->   openbsd/amd64: github.com/yuya-takeyama/gox-test
-->     windows/386: github.com/yuya-takeyama/gox-test
-->   windows/amd64: github.com/yuya-takeyama/gox-test
-->     freebsd/arm: github.com/yuya-takeyama/gox-test
-->      netbsd/386: github.com/yuya-takeyama/gox-test
-->    netbsd/amd64: github.com/yuya-takeyama/gox-test
-->      netbsd/arm: github.com/yuya-takeyama/gox-test
-->       plan9/386: github.com/yuya-takeyama/gox-test
$ tree
.
├── gox-test_darwin_386
├── gox-test_darwin_amd64
├── gox-test_freebsd_386
├── gox-test_freebsd_amd64
├── gox-test_freebsd_arm
├── gox-test_linux_386
├── gox-test_linux_amd64
├── gox-test_linux_arm
├── gox-test_netbsd_386
├── gox-test_netbsd_amd64
├── gox-test_netbsd_arm
├── gox-test_openbsd_386
├── gox-test_openbsd_amd64
├── gox-test_plan9_386
├── gox-test_windows_386.exe
├── gox-test_windows_amd64.exe
└── main.go

0 directories, 17 files
```

gox には `-output` オプションがあり、ここに `text/template` 形式のフォーマットで出力先を指定できます。

```
 gox -output "pkg/{{.OS}}_{{.Arch}}/{{.Dir}}"
Number of parallel builds: 4

-->      darwin/386: github.com/yuya-takeyama/gox-test
-->    darwin/amd64: github.com/yuya-takeyama/gox-test
-->       linux/386: github.com/yuya-takeyama/gox-test
-->     linux/amd64: github.com/yuya-takeyama/gox-test
-->       linux/arm: github.com/yuya-takeyama/gox-test
-->     freebsd/386: github.com/yuya-takeyama/gox-test
-->   freebsd/amd64: github.com/yuya-takeyama/gox-test
-->     openbsd/386: github.com/yuya-takeyama/gox-test
-->   openbsd/amd64: github.com/yuya-takeyama/gox-test
-->     windows/386: github.com/yuya-takeyama/gox-test
-->   windows/amd64: github.com/yuya-takeyama/gox-test
-->     freebsd/arm: github.com/yuya-takeyama/gox-test
-->      netbsd/386: github.com/yuya-takeyama/gox-test
-->    netbsd/amd64: github.com/yuya-takeyama/gox-test
-->      netbsd/arm: github.com/yuya-takeyama/gox-test
-->       plan9/386: github.com/yuya-takeyama/gox-test
$ tree
.
├── main.go
└── pkg
    ├── darwin_386
    │   └── gox-test
    ├── darwin_amd64
    │   └── gox-test
    ├── freebsd_386
    │   └── gox-test
    ├── freebsd_amd64
    │   └── gox-test
    ├── freebsd_arm
    │   └── gox-test
    ├── linux_386
    │   └── gox-test
    ├── linux_amd64
    │   └── gox-test
    ├── linux_arm
    │   └── gox-test
    ├── netbsd_386
    │   └── gox-test
    ├── netbsd_amd64
    │   └── gox-test
    ├── netbsd_arm
    │   └── gox-test
    ├── openbsd_386
    │   └── gox-test
    ├── openbsd_amd64
    │   └── gox-test
    ├── plan9_386
    │   └── gox-test
    ├── windows_386
    │   └── gox-test.exe
    └── windows_amd64
        └── gox-test.exe

17 directories, 17 files
```