Summary

This PR introduces a new ASDF_ environment variable called ASDF_FORCE_INSTALL which can be set to true to force a (re)install.

Taking Ruby as an example, it's desirable to want to re-install (instead of uninstall and install) when you want to apply a new set of Ruby patches, but don't want to lose the currently installed Ruby gems.

@Stratus3D Please may I have your thoughts on this approach? I haven't added documentation or updated any required tests as yet as I wanted to see if you would be in support of such an idea.

I think by the very least, the reinstall should be implemented with some sort of flag for consistency. Going through #244, it seems this has been mentioned here.

I don't have any comments about the approach, but it seems there were some issues with transactional consistency as mentioned in that issue. There were some approaches mentioned here, and I wonder if it would be better implementing one of them rather than going with a naive approach. Because I think in most cases, the directory would already exist and we would want to reinstall over that?