Add --maxWorkers switch

In fixing npm#19170, rvagg/node-worker-farm#65 falls back to basically
disabling multiple workers, so the user may want a switch to override.

lib/install/action/extract.js

@@ -36,10 +36,15 @@ const ENABLE_WORKERS = process.platform === 'darwin'
 
 extract.init = () => {
   if (ENABLE_WORKERS) {
-    workers = workerFarm({
+    var workerOptions={  
       maxConcurrentCallsPerWorker: npm.limit.fetch,
       maxRetries: 1
-    }, WORKER_PATH)
+    }
+    var maxWorkers=parseInt(npm.config.get("maxWorkers"))
+    if (maxWorkers) {
+      workerOptions.maxConcurrentWorkers = maxWorkers
+    }
+    workers = workerFarm(workerOptions, WORKER_PATH)
   }
   return BB.resolve()
 }