VCS-friendly output default template

[mcandre]

A template like "bin/{{.Dir}}/{{.OS}}/{{.Arch}}/{{.Dir}}" could be more helpful than {{.Dir}}_{{.OS}}_{{.Arch}}:

• Easier to gitignore port files by the root directory (e.g. bin/)
• Same basename for each port file ({{.Dir}}) for easier installation and invocation
• Files organized in a fashion suitable for easy zip -r'ing