allure plugin for robotframework-seleniumlibrary

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#### I'm submitting a ...
- [ ] bug report
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This could be an good example or part of a robot adapter
```python
import allure
from SeleniumLibrary import ScreenshotKeywords
from SeleniumLibrary.base import keyword, LibraryComponent

class AllureSeleniumPlugin(LibraryComponent):

@keyword
def capture_page_screenshot(self, filename='selenium-screenshot-{index}.png'):
path = ScreenshotKeywords(ctx=self.ctx).capture_page_screenshot(filename)
allure.attach.file(path, attachment_type=allure.attachment_type.PNG)
```

```robot
Library SeleniumLibrary plugins=AllureSeleniumPlugin
```

提案されている Python AllureSeleniumPlugin の例と、その capture_page_screenshot エントリーポイントから始めます。Robot Framework Library の設定が plugins=AllureSeleniumPlugin をどのように渡すかを確認し、その後、統合に必要な範囲を判断します。Done では、取得したページのスクリーンショットを Allure レポートに添付できる、実用的な SeleniumLibrary プラグインを提供する必要があります。

索引モデルが issue の本文から書いたものです。