mason cache-clear command

[springmeyer]

There is a ./mason remove <package> <version> command that un-installs a package from mason_packages/<platform>/<package name>/<version>/. However if you remove a package and then re-install it you will get the same identical package from the cache at ./mason_packages/.binaries (instead of downloading the version from remote).

This ticket stands to remind us to:

• document this behavior
• consider adding a command to make this cache clearing easier (to avoid having to rm -rf ./mason_packages/.binaries/....

/cc @AllieOop