Console Runner

• A new option, --list-extensions, will display all the engine extensions that
  have been installed by the engine.

Issues Resolved

• 1623 NUnit 3.4 is not integrated with TeamCity
• 1626 NUnit.ConsoleRunner is not picking up NUnit.Extension.NUnitV2ResultWriter
• 1628 Agent's process stays in memory when it was failed to unload AppDomain
• 1635 Console option to list loaded extensions