feat: allowing find the closest version if the exact version is not found

[bsalesc]

Summary

This is a way to allow you find the closest version in a non semantic way, if plugins are using semantic version is makes even easier.

I was going to open a feature request but there seem to be one opened here #352.

This makes easier for users that have the below cases:

$ asdf list python
  2.7.18
  3.1.5
> 3.2.0
  3.2.1

In which I could asdf global python 2 and as I've 2.7.18 installed and it's the only one that starts with 2, then that's the version that will be used, if I want 3, I need to be more specific, asdf global python 3.1, asdf global python 3.2.0 or asdf global python 3.2.1. Otherwise it'll fallback and complain saying version {VERSION} is not installed for {PLUGIN}.

Fixes: #352

Other Information

I've added tests for the "when is not found a single version" and "when it's found the closest version", let me know if there is need to have more tests.

[RobLoach]

This is very handy. Thanks.