I want to addEnvironment(name, value) via code instead of xml file

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#### I'm submitting a ...
- [ ] bug report
- [x] feature request
- [ ] support request => Please do not submit support request here, see note at the top of this template.

#### What is the current behavior?
I can't found add environment function in allure

#### If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

#### What is the expected behavior?
I want to addEnvironment(name, value) via code

#### What is the motivation / use case for changing the behavior?
I want to add environment to show my test line info, and I use jenkins to executor our testcase

#### Please tell us about your environment:

- Allure version: 2.5.0
- Test framework: pytest@3.6.3
- Allure adaptor: allure-pytest@2.5.0

#### Other information

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. e.g. detailed explanation, stacktraces, related issues, suggestions
. how to fix, links for us to have more context, eg. Stackoverflow, Gitter etc
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首先定位 allure-pytest 适配器中的环境 XML 处理逻辑和公共 API；该 issue 未指定文件或测试。确定基于代码的环境条目应放置的位置，并将完成定义为：能够通过适配器添加一个 name/value 对，并在生成的 Allure 报告中验证它。

由索引模型根据 Issue 内容生成。