Allow `type` fields in metafiles to hide type prompt

This closes #603

lib/read-metadata.js

@@ -40,11 +40,21 @@ async function readMetaData(
     }
   }
 
+  // We can remove this once the prompt for choosing `--npm` or `--docker` is gone
+  if (pkg && pkg.now && pkg.now.type) {
+    type = nowConfig.type
+  }
+
+  // The same goes for this
+  if (nowConfig && nowConfig.type) {
+    type = nowConfig.type
+  }
+
   if (!type) {
     type = await determineType(path)
 
     // Both `package.json` and `Dockerfile` exist! Prompt the user to pick one.
-    // We can remove this soon (details are internal)
+    // We can remove this soon (details are internal) - also read the comment paragraph above
     if (type === 'docker' && (pkg && dockerfile)) {
       const err = new Error(
         'Ambiguous deployment (`package.json` and `Dockerfile` found). ' +