Clarify gitignore patterns

This commit updated the gitignore file to clarify.
Clarify the test class

This commit clarified the test class name
from AutoDataTheoryCommandCustomizationTest
to AutoDataTheoryCommandExecuteTest.
Not to reference AutoFixture.Xunit

This commit made not to reference AutoFixture.Xunit using reflection.
Remove AutoFixture.Xunit package

This commit removed the AutoFixture.Xunit package from the project.
Simplify the AutoDataTheoryCommandTest code

This commit reformatted to simplify the AutoDataTheoryCommandTest code.
Increment pre-release major version

This commit is to indicate that removing the AutoFixture.Xunit reference
is breacking change. Instead of it, AutoFixture.Contrib resolves it
through reflection.