Env variable to set cool-down-period

Hi!

I've noticed that there is a new setting for setting dependency cooldown period: https://pipenv.pypa.io/en/latest/pipfile.html#cool-down-period

However there is no env variable to control this globally.

Can you add env variable for this setting so it is possible to set it once instead of setting it in every Pipfile?

If you accept PRs, and you think this is a valid feature I can provide a PR.

Thanks!