asdf plugin

asdf

Add a plugin from the plugin repo OR, add a Git repo as a plugin by specifying the name and repo url

List installed plugins. Optionally show git urls and git-ref

Update a plugin to latest commit on default branch or a particular git-ref

Name	Description
`add`	Add a plugin from the plugin repo OR, add a Git repo as a plugin by specifying the name and repo url
`list`	List installed plugins. Optionally show git urls and git-ref
`remove`	Remove plugin and package versions
`update`	Update a plugin to latest commit on default branch or a particular git-ref