add cache commands "ls" and "rm"

[fritzy]

This adds the ability to list and delete cached package downloads.

ls [<package>@<version query>]
Lists all cache keys, or keys related to certain package specs. Filtered results are best effort based on both existing packuments in cache (further resolving package keys) as well as a fallback to filtering tgz packages based on URL.
Any number of packages can be specified.
Packuments cache keys match on any version spec.

rm [<key>] (alias of clean)
Removes any number of keys

I extended clean to accept any number of cacache keys. Zero arguments still removes everything and requires --force.

[fritzy]

Additionally I fixed unscoped packages from matching scoped packages in npm cache ls --package name

[fritzy]

I've finished the test coverage, so this is now ready for review.

[nlf]

in addition to the comments above, one last thing:

the params static getter should have 'package' added to it, that'll make sure that when docs are generated it will list the --package flag as one in use by this command

[nlf]

looks great!