添加clean子命令

[voidint]

假设待编译的项目结构类似这样

./go
└── cmd
    ├── cmd1
    │   └── main.go
    ├── cmd2
    │   └── main.go
    └── cmd3
        └── main.go

并且在gbb.json中指定的tool是go build，那么编译后必然在每个cmd子目录下留下一个可执行文件。原生的go clean命令仅支持清除当前目录下的编译产物，gbb clean将遍历所有的子命令目录并清除编译产物。

gbb clean命令将支持go clean命令的所有命令行选项：

The -i flag causes clean to remove the corresponding installed
archive or binary (what 'go install' would create).

The -n flag causes clean to print the remove commands it would execute,
but not run them.

The -r flag causes clean to be applied recursively to all the
dependencies of the packages named by the import paths.

The -x flag causes clean to print remove commands as it executes them.