We introduce the `plugin` nu sub command (`nu plugin`) with basic plugin
loading support. We can choose to load plugins from a directory. Originally
introduced to make integration tests faster (by not loading any plugins on startup at all)
but `nu plugin --load some_path ; test_pipeline_that_uses_plugins_just_loaded` does not see it.
Therefore, a `nu_with_plugins!` macro for tests was introduced on top of nu`s `--skip-plugins`
switch executable which is set to true when running the integration tests that use the `nu!` macro now..
