Support prune when installing/updating environments

[AlbertDeFusco]

A recent update to libmamba fixes conda env update --prune. This opens up the opportunity to provide a rollback for conda-project.

See #8 for the most recent discussion. One thing to be careful with is protecting the exactness of the match between the installed env and the lock. This is maintained currently by 1) removing the installed env and 2) re-installing all packages from the explicit file generated from the yaml lock file.

Perhaps a prune-enabled environment update is a less strict interpretation of matching between installed env and lockfile.