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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まず、Python の @allure.step デコレーターが生成されたレポートに引数を記録する方法を追跡し、現在 workaround として使用されている with allure.step() コンテキストマネージャーと比較します。関連するテストを特定する前に、大きな引数値に対する切り詰めの上限と期待される動作を定義します。Step パラメーターが大きすぎる場合は制限され、attachments では引き続き完全な内容を保持できれば完了です。

索引モデルが issue の本文から書いたものです。