feat(clean): hide npm cache clean behind --force

lib/cache.js

@@ -19,7 +19,7 @@ cache.usage = 'npm cache add <tarball file>' +
               '\nnpm cache add <tarball url>' +
               '\nnpm cache add <git url>' +
               '\nnpm cache add <name>@<version>' +
-              '\nnpm cache clean [<pkg>[@<version>]]' +
+              '\nnpm cache clean' +
               '\nnpm cache verify'
 
 cache.completion = function (opts, cb) {
@@ -63,6 +63,9 @@ cache.clean = clean
 function clean (args) {
   if (!args) args = []
   const cachePath = path.join(npm.cache, '_cacache')
+  if (!npm.config.get('force')) {
+    return BB.reject(new Error("As of npm@5, the npm cache self-heals from corruption issues and data extracted from the cache is guaranteed to be valid. If you want to make sure everything is consistent, use 'npm cache verify' instead.\n\nIf you're sure you want to delete the entire cache, rerun this command with --force."))
+  }
   // TODO - remove specific packages or package versions
   return rm(cachePath)
 }