[FR] Provide current benchmark information

**Is your feature request related to a problem? Please describe.**
Currently at GoogleTest we use the name of the test to create a unique folder per test where the tests should write (so that the folder names are unique, but also easy to find when debugging for example). So we are using the data provided by `testing::UnitTest::GetInstance()->current_test_info()` in a helper class. Sadly, there is nothing similar in the Benchmark library.

**Describe the solution you'd like**
Provide a way to retrieve information about the current running benchmark - similar to [`TestParamInfo` of GoogleTest](http://google.github.io/googletest/reference/testing.html#TestParamInfo).