Invoke a post install script after installing a new version

There is https://github.com/tpope/rbenv-ctags which that generate tags automatically for ruby. It would be nice to setup similar functionality with asdf. Having the installer invoke a script if a user has defined one, similar to the default gems, seems like it could support other use cases as well.

Start by locating the installer entry point and the existing default-gems handling referenced in the issue. Trace when a new Ruby version finishes installing and define the user-script configuration and execution behavior; done means the script runs after installation without breaking installs.

Written by the indexing model from the issue text.