* Implement info command
* Remove duplicate and unneeded BATS tests from `main_test.go`
* remove download dir for plugin when removing plugin
* update plugins.Add function so downloads directory is created
* 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
Rename custom scripts for `plugin-add` and `plugin-remove` phases in plugins to make clear when they are executed:
`plugin-add` -> `post-plugin-add`
`plugin-remove` -> `pre-plugin-remove`
When checking if a plugin is installed (to display a flag) it is not
necessary to loop through all installed plugins, since we already have
a name we can check directly.
Also expand test case to test this code path too.
When a shim is created, add plugin metadata so we can later know which shims belong to which plugins, this will help aid with removing unused shims on uninstall. See #67
```
# asdf-plugin: ${plugin_name}”
```
Thanks to @duijf for the metadata proposal.
* Rename `asdf which` -> `asdf current`
* Output `set by $path` with current command
* Use dummy plugin in current_command test
* Hide "set by" message if derived from legacy file
