A flag to display a list of available parameters

_From @sebawita on December 4, 2017 18:50_

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Area
```
- [ ] devkit
- [x] schematics
```

### Desired functionality
Currently the only way to check the available parameters for a schematics is by looking into its json schema file.
It would be great to have a way in the CLI to display the list of params, like this:
```schematics @schematics:schematic --schema```

_Copied from original issue: angular/devkit#311_

首先跟踪 schematics CLI 的入口点，以及它如何读取每个 schematic 的 JSON schema。运行建议的 `schematics @schematics:schematic --schema` 命令，同时检查现有的 CLI 行为。完成的标准是，该 flag 能显示某个 schematic 可用的参数，而无需用户检查其 schema file。

由索引模型根据 Issue 内容生成。