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

Riproduci l’esempio usando Python subTest, pytest, pytest-subtests e allure-pytest 2.6.2, quindi ispeziona il report Allure generato. Il lavoro è completato quando nel report compare un subTest fallito e questo causa il fallimento del test complessivo, mentre le soft assertions successive continuano a essere eseguite.

Scritto dal modello di indicizzazione a partire dal testo della issue.