Please add "pyenv list" - Improve usability for listing available versions

[newearthmartin]

Please make a first level command for seeing which python versions are available. I suggest pyenv list.

Every time I need to see available versions, I have to google it because I don't remember that it is pyenv install -l and it does not show up in pyenv --help.

It is one of the most common use cases so it should be at the top level, easily identifiable in pyenv --help.

[onaforeignshore]

I added a comment to a very old pull request for compsys completion that would describe the flags in the completion so it may be easier to remember. If you want to add it to your environment, take a look here