Add stdout and stderror capturing with python behave

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

#### What is the current behavior?
Output from stdout and stderror is not captured for the report when using allure-behave.

#### What is the expected behavior?
Stdout and stderror output should be available when viewing Overview for the test.

#### What is the motivation / use case for changing the behavior?
To have an easy way to see the test-related output for future debugging/bug reporting/etc.

#### Please tell us about your environment:
- Allure version: 2.4.1
- Test framework: behave@1.2.6
- Allure adaptor: allure-behave@2.6.5

Start by tracing the allure-behave integration for behave@1.2.6 and how test output is currently passed to reports. Verify that the Overview for a test exposes both stdout and stderr, using the stated Allure and allure-behave versions as the reproduction environment.

Written by the indexing model from the issue text.