* Add go-git as a dependency
* Remove plugins dir from gitignore
* Move config code to config package
* Create validatePluginName function
* Create PluginDirectory function
* Create PluginExists function
* Implement PluginAdd function
* Add testify as a dependency
* Write test for PluginAdd happy path
As the proposal for asdf-nodejs to [validate OpenPGP
signatures](https://github.com/asdf-vm/asdf-nodejs/pull/25) has been
approved and merged I guess the design is sound and other plugins can
follow.
Related to: #159
