Limit step parameter content size in allure report

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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?
Given following example
```python
import allure

def test_example():
big_arg = 'abcdef ' * 1000
example_step(big_arg)

@allure.step("Example Step")
def example_step(big_arg):
pass
```
In allure report for above test for `example_step` step argument `big_arg` value is conveniently included in report. Even if value is quite large.

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

#### What is the expected behaviour?

#### What is the motivation / use case for changing the behaviour?
There are valid cases where argument(s) passed to `allure.step` function is quite large and in my opinion it does not make sense to include all of it allure report. Perhaps instead it could be truncated to some reasonable length?

In case user wants to keep all contents of specific variables/arguments he or she can always use attachment feature.

In some cases where allure step functions are dealing with quite large text chunks this increases allure report size considerably. For now as a workaround I have to avoid using `@allure.step` decorator and instead use `with allure.step()` context manager inside step function.

#### Please tell us about your environment:

- Allure version: 2.7.0
- Test framework: pytest@3.6.3
- Allure adaptor: allure-pytest@2.5.0

#### Other information

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. how to fix, links for us to have more context, eg. Stackoverflow, Gitter etc
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首先追踪 Python @allure.step 装饰器如何在生成的报告中记录参数，并将其与当前作为 workaround 使用的 with allure.step() 上下文管理器进行比较。在确定相关测试之前，先定义对较大参数值的截断限制和预期行为。完成的标准是：超大的 Step 参数受到限制，同时 attachments 仍能保留完整内容。

由索引模型根据 Issue 内容生成。