* Refactor config package so default settings are always used with config file not loaded
* Update `findVersionsInDir` to check legacy files
* Refactor functions in internal/resolve package to return `ToolVersion` structs instead of version slices
* Refactor `findVersionsInEnv` function to return env variable name
* Create `resolve.Version` function
* 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