Merge pull request #6 from m0sth8/master

Fix download url
Add an 'install only' option

This will run the download & extract part of the script, without
dropping you into a subshell
Add a way to quiet messages

This way I can run in --install-only mode and easily
get the GOROOT while running goenv from a script
don't do this
breaking change -- change --version to --go-version