Feature Request: check versions before install

[tir38]

I can run the installer twice in a row and it'll duplicate much of the work. It would be helpful if ruby-install could check if a version is installed already before running through the whole install process.

[postmodern]

We already have a --no-reinstall flag which checks for the existence of $install_dir/bin/ruby, prints a warning and exits. We could make this behavior the default behavior, and require --reinstall or --overwrite option flags to force ruby-install to re-install over an existing ruby install.

[tir38]

I think adding something to the README would be good enough, although I see there is already an open issue to do that: #266

[todd-a-jacobs]

I have a Fish function that's easily converted to Bourne-like shells (mostly the array logic and regexp pattern matching currently leverage Fish rather than external utilities like grep or known Bashisms) that could check the current list of Rubies reported by chruby against the latest as reported by ruby-install. I'm deliberately going through the existing utilities rather than looking at the filesystem directly as a convenience and a useful (to me) abstraction that keeps chruby and ruby-install as the authoritative source of versions under their management. Happy to share the relevant snippets so people can kick the tires, or to try to add the code directly as a PR if it's a desirable feature.