* Create `plugins.New` function, updating existing code to use it
* Add another test for `hook.Run` function
* Enable `plugin_add_command.bats` tests for Go implementation of asdf
* Add `RunCallback` method to `Plugin` struct
* Update `plugins.Add` function to run `post-plugin-add` plugin callback script
* Handle Bash expression and scripts properly in `execute` package so `$@` is always set