Non blocking fish prompt

[furudean]

I propose this as a less intrusive alternative to #451. This can be dropped in as a stop gap to ease some frustration for fish users. I think the philosophy in #451 is a better approach if we can figure out all the difficult bits.

This PR will make pyenv run before the command executes and won't block the prompt from showing, which feels a lot better to use.

See also #338

[native-api]

There are side effects: at prompt time, changes made by activating an environment won't yet be in effect.

This would affect things like interactive completion.

[furudean]

Fair enough