First-Use Experience Enhancement

This PR includes significant changes to the way go-plus discovers your go installation and the executables that it uses to complete its work.

Gone is the need to explicitly configure go-plus with paths to each tool that you use. go-plus will now:

- [x] Inspect the path and also the default installation location(s) for your platform to find your go installation
- [x] Inspect your environment for the GOPATH and fall back to a configured GOPATH to find executables (e.g. goimports); multiple items in a GOPATH are supported
- [x] Run the command pipeline asynchronously, in parallel
- [x] Aggregate any warnings consistently
- [x] Support HomeBrew installed go as a first-class citizen
- [x] Support any go installed and available on your path
- [ ] Fetch any missing tools with `go get <path to tool>` – which can be disabled
- [ ] Explicitly allow use of goimports (instead of gofmt), because let's face it, there's only two sheriffs in town