Support python subtests and show failures of subtests in the report

#### I'm submitting a ...
- [x] feature request

#### What is the current behavior?
I'm using soft asserts in my tests. I've implemented those using the subTest python lib.
Example:
```
def __soft_assert_equal(self, a, b, msg):
with self.subTest():
self.assertEqual(a, b, msg)
```
This way if a soft assert fails the tests scripts will go on and execute the next asserts.
I'm running the tests with pytest and using this plugin to show the soft assert failures in the pytest output https://github.com/pytest-dev/pytest-subtests.

However the Allure reports do not show the failures from the subTest and show the test as passed.
Could this be implemented, so Allure reports will show if a assert from a subTest has failed and fail the test in that case? Thanks

#### Please tell us about your environment:

- Allure version: 2.11.0
- Test framework: Python 3.7.3, pytest-4.4.1
- Allure adaptor: allure-pytest-2.6.2

Python subTest、pytest、pytest-subtests、allure-pytest 2.6.2 を使って例を再現し、その後生成された Allure レポートを確認します。レポートに失敗した subTest が表示され、それによってテスト全体が失敗しつつ、後続の soft assertions も引き続き実行されれば作業は完了です。

索引モデルが issue の本文から書いたものです。