auto-args for targets?

It wouldn't be too hard to auto-generate a CLI for each target so the user could pass in positional args or flags per arg.

e.g.

```go
func Build(OS string, ARCH string) error {
     // build with GOOS and GOARCH
}

$ mage build -os darwin -arch amd64
building with GOOS=darwin GOARCH=amd64
```


