Change CLI to take packages as parameters, like go list.

- Add `--stdin` flag to read package list from stdin, as before.
- Minor refactoring to improve code, reduce dot imports.

With this change, `gostatus` becomes a lot more like other go tools, namely like `go list`, `go build`, `go install`. It accepts a list of import paths or import path patterns after flags.

E.g.,

```bash
go list .
go list all
go list github.com/...
go list ...shurcooL/gostatus
go list ...util... ...tool...

gostatus .
gostatus all
gostatus github.com/...
gostatus ...shurcooL/gostatus
gostatus ...util... ...tool...
```

Closes #17.