Default format for gherkin tags

#### 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?
If no allureConfig.json with tag patterns specified in it, all gherkin tags are converted to Allure tags.

#### What is the expected behavior?
We should provide reasonable defaults. For example, we might convert `allure.suite:My-suite` to the label `suite=My-suite`.

#### What is the motivation / use case for changing the behavior?
The motivation is to make the tags API of Allure SpecFlow usable without configuration.

#### Please tell us about your environment:

- Allure version: 2.1.0
- Test framework: testng@6.8
- Allure adaptor: allure-testng@2.0-BETA8
- Generate report using: allure-maven@2.18

#### Other information

That will be a breaking change. We might provide a feature switch in allureConfig.json for those who don't want default patterns for some reasons.

Start by locating the SpecFlow Gherkin-tag conversion and the handling of allureConfig.json. Define and verify reasonable default tag patterns, including the suite example, while checking how a configuration switch could preserve existing behavior; done means tags convert as specified without configuration and the breaking-change behavior is documented or tested.

Written by the indexing model from the issue text.