envy clean --all added

This release introduces a new feature allowing users to clean all envy edit's or sync's at once, as opposed to individually.

For example, assume that a user has made edits to both foo and bar in some virtualenv, call it env. Previously, in order to return the foo and bar site-packages to their original state, the user would need to run both
envy clean foo and envy clean bar.

Now, the user can accomplish this in one step by running envy clean --all.