Some of the Go tests actually cloned remote repos, which meant slow tests, and tests failures if I was working offline. These changes allow all Go tests to run offline. The test code now clones local Git repos instead of remotes ones.
* Add otiai10/copy as a dependency
* Create repotest package for test code that needs to work with Git repos
* Add back missing test script for execute package
* Update git, pluginindex, and plugins package tests so they work offline
* 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
* rename commands package to cmd
* add go-ini as a dependency
* write code to parse asdfrc file into Settings struct
* add go-envconfig as a dependency
* add go-homedir as a dependency
* implement first version of LoadConfig function to load environment variables into Config struct
