Support for multiple entries on GOPATH

* Respects any preset GOPATH
* Now will work for third party packages in a default go workspace and separate code in another workspace (as described in [third party packages](https://github.com/golang/go/wiki/GOPATH#third-party-packages) on the go wiki). This is now the recommended approach
* goenv cd will automatically add the workspace for the project
* goenv cd completion will restrict to the current workspace as detected