remove stale dependencies

[Alloyed]

this would either be a flag to purge, or a custom command (clean?). it might make sense to remove purge, or replace it with rm -rf rocks/. although having it adds a sense of discoverability, you can read the help and know how to empty a tree

[pablomayobre]

My suggestion is the following:

• clean should be a command that gives you a clean state (currently this command is called purge)
• prune should be a command that cleans the unneeded/stale dependencies (this is the behaviour of npm prune)
• deps --upgrade should be a command to install new dependencies and remove unneeded/stale ones (this is not actually needed but could be a nice upgrade)

Note that I have removed purge in favour of clean, and added prune which has the described behaviour.