Add support for zip images in select image dialog

The "Select Image" dialog now permits the user selecting zip files. Once
the zip file is selected, `resin-zip-image` scans the archive to ensure
its validity. If its not valid, an error alert is shown and nothing is
selected.

lib/src/dialog.js

@@ -18,6 +18,7 @@
 
 const electron = require('electron');
 const Bluebird = require('bluebird');
+const zipImage = require('resin-zip-image');
 
 /**
  * @summary Open an image selection dialog
@@ -25,7 +26,10 @@ const Bluebird = require('bluebird');
  * @public
  *
  * @description
- * Notice that by image, we mean *.img/*.iso files.
+ * Notice that by image, we mean *.img/*.iso/*.zip files.
+ *
+ * If the user selects an invalid zip image, an error alert
+ * is shown, and the promise resolves `undefined`.
  *
  * @fulfil {String} - selected image
  * @returns {Promise};
@@ -41,15 +45,27 @@ exports.selectImage = function() {
       properties: [ 'openFile' ],
       filters: [
         {
-          name: 'IMG/ISO',
+          name: 'IMG/ISO/ZIP',
           extensions: [
+            'zip',
             'img',
             'iso'
           ]
         }
       ]
     }, resolve);
-  }).get(0);
+  }).get(0).then(function(file) {
+    if (zipImage.isZip(file) && !zipImage.isValidZipImage(file)) {
+      electron.dialog.showErrorBox(
+        'Invalid zip image',
+        'Etcher can only open Zip archives that contain exactly one image file inside.'
+      );
+
+      return;
+    }
+
+    return file;
+  });
 };
 
 /**

package.json

@@ -61,7 +61,7 @@
     "lodash": "^4.5.1",
     "ngstorage": "^0.3.10",
     "resin-image-write": "^2.0.5",
-    "resin-zip-image": "^1.1.0",
+    "resin-zip-image": "^1.1.1",
     "sudo-prompt": "^2.2.0",
     "trackjs": "^2.1.16",
     "umount": "^1.1.1",