Limit step parameter content size in allure report

[//]: # (
. Note: for support questions, please use Stackoverflow or Gitter**.
. This repository's issues are reserved for feature requests and bug reports.
.
. In case of any problems with Allure Jenkins plugin** please use the following repository
. to create an issue: https://github.com/jenkinsci/allure-plugin/issues
.
. Make sure you have a clear name for your issue. The name should start with a capital
. letter and no dot is required in the end of the sentence. An example of good issue names:
.
. - The report is broken in IE11
. - Add an ability to disable default plugins
. - Support emoji in test descriptions
)

#### 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

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

Beginne damit nachzuverfolgen, wie der Python-Dekorator @allure.step Argumente im generierten Report erfasst, und vergleiche dies mit dem Kontextmanager with allure.step(), der derzeit als Workaround verwendet wird. Lege das Kürzungslimit und das erwartete Verhalten für große Argumentwerte fest, bevor du die relevanten Tests identifizierst. Als erledigt gilt die Aufgabe, wenn übergroße Step-Parameter begrenzt sind, während Attachments weiterhin den vollständigen Inhalt bewahren können.

Vom Indexierungsmodell aus dem Issue-Text verfasst.