allure.environment is missing in allure.py

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

#### What is the current behavior?
Based on documentation [here](https://github.com/allure-framework/allure-pytest) to apply some environment configurations we should use allure.environment(report='Allure report', browser=u'Я.Браузер') syntax. But in allure.py environment is missing. Before we have to use some environment.properties file in report folder to apply variables to the report. New functionality is missing.
Content of allure.py:
```
from allure_commons._allure import label
from allure_commons._allure import severity
from allure_commons._allure import tag
from allure_commons._allure import epic, feature, story
from allure_commons._allure import link
from allure_commons._allure import issue, testcase
from allure_commons._allure import Dynamic as dynamic
from allure_commons._allure import step
from allure_commons._allure import attach
from allure_commons.types import Severity as severity_level
from allure_commons.types import AttachmentType as attachment_type

__all__ = [
'label',
'severity',
'tag',
'epic'
'feature',
'story',

'link',
'issue',
'testcase',

'step'

'dynamic'

'severity_level',

'attach',
'attachment_type'
]

```

#### 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?

#### What is the motivation / use case for changing the behavior?

#### Please tell us about your environment:

- Allure version: 2.2.1
- Test framework: pytest@3.5
- Allure adaptor: allure-pytest@2.1.0b1

#### Other information

[//]: # (
. e.g. detailed explanation, stacktraces, related issues, suggestions
. how to fix, links for us to have more context, eg. Stackoverflow, Gitter etc
)

从 allure.py 开始，将其导出的 API 与 issue 中链接的 allure-pytest 文档进行比较，重点关注文档化的 allure.environment 语法。检查现有的 issue 讨论以了解上下文，并确定预期的公共 API；完成的标准是，文档化的环境配置可以通过 allure.py 使用，并且行为如描述所述。

由索引模型根据 Issue 内容生成。