Alias support

A discussion about aliases has started in https://github.com/asdf-vm/asdf-elixir/pull/63.
As this is not directly related to asdf-elixir but rather to asdf, let's have the discussion here.

I personally only use "regular" versions so I do not have a strong opinion about this but I get the point.

I am not exactly sure how what the `--alias` flag is about though?
Could somebody please propose what he thinks would be a good way for asdf to handle aliases?

Read the alias discussion in this issue and the referenced asdf-elixir pull request #63 first. No file, test, entry point, or agreed behavior is named; the work is not ready to start until the alias and --alias requirements are defined.

Written by the indexing model from the issue text.