Add support for all configuration elements via RunSettings

[bradwilson]

The configuration file values should all be passable via RunSettings. This would allow developers to either create .RunSettings files for their projects which exclusively use xunit.runner.visualstudio to run tests and/or allow them to use a .RunSettings file (or dotnet test command line switches) to override values that are normally provided via xunit.runner.json.

The order of precedence from highest to lowest will be:

• VSTest environmental options DisableAppDomain and DisableParallelization
• Settings from .RunSettings and/or dotnet test command line switches
• Settings from xunit.runner.json
• Default values

Related: #189 (comment) @tig

[bradwilson]

Documention page: https://xunit.net/docs/runsettings