Add VoltSigner to tools (#1175)

I posted this to the dev mailing list on April 30. As there were no replies, I hope it's ok to move to the next step:

We’ve got a new tool we’d like to share with the Cordova community. It facilitates the creation of signing certificates for Android and iOS.

https://voltsigner.com

- It’s free
- No login is required
- We’re in the process of making of making the source public on GitHub
- Runs completely in the browser
- Generates Android and iOS .p12 files on a Windows or MacOS computers.
- Certificates are made 100% locally - passwords and files are not shared to the cloud.

Author: gwhenne

www/_data/tools.yml

@@ -90,3 +90,12 @@
         Modern replacement for PhoneGap Build. Upload project and certificates,
         then download builds (or upload to store). Uses latest 
         Apache Cordova and SDKs. There's nothing to install: just sign up and submit.
+        
+-   name: VoltSigner
+    image: voltsigner.png
+    url: https://voltsigner.com
+    description: >
+        Free service to create Android and iOS signing certificates.
+        Run completely in your local browser - nothing gets uploaded.
+        Compatible with all build tools.
+        Notably, it can produce iOS certificates on Windows.