Allow the use of custom subclasses of StepDefinitionBaseAttribute.

[dnperfors]

These changes allow for creation and usage of custom stepdefinition attributes like:

public class GivenAndWhenAttribute : StepDefinitionBaseAttribute
{
    public GivenAndWhenAttribute(string regex)
        : base(regex, new[] { StepDefinitionType.Given, StepDefinitionType.When })
    {
    }
}

Closes #1932

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code. (most of the time mandatory)
• I have added an entry to the changelog. (mandatory)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[SabotageAndi]

Thanks for your contribution to SpecFlow. Sorry, that it took us so long to merge this PR.
I updated the changelog to the current state.

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