envy reset feature

This release introduces envy reset. This is useful for dropping stale backups of older package versions without re-applying the changes to the live site-package in your virtualenv.

To drop a backup for a specific package, say, bar, run:

(foo)$ envy reset bar
dropping saved envie for bar

To remove all backups for a given environment, call it foo, run:

(foo)$ envy reset --all
dropping saved envie for bar
dropping saved envie for baz
dropping saved envie for requests