Ability to configure benchmark class name in outputs

[cdonke]

Implementation of attribute to rename class.

Closes #1447

[AndreyAkinshin]

Here are some ideas from my side:

1. I agree with @timcassell, BenchmarkName can be a confusing name in such a situation. I would suggest BenchmarkDescription.
2. If we introduce such an attribute, it makes sense to "extract" the Description property from the [Benchmark] attribute to be consistent and annotate both methods and classes with a new attribute.
3. Currently, a benchmark description overrides the method name. I would suggest splitting the original class/method names and their descriptions in the exporters so that we always have access to the original names during csv/json/xml/etc parsing.
4. In the Summary table I would suggest using "adaptive" default column that shows a description if it's available and the original name otherwise.