Add Tower Git client (#39)

* Add Tower Git client

* Enhance “Tower” description and URI

_data/perks.yml

@@ -207,3 +207,8 @@
   uri: "https://www.querystorm.com/"
   description: "QueryStorm brings SQL and C# into Excel. It enables querying Excel tables with SQL/LINQ and allows rapid prototyping with C#/Excel. Microsoft MVP's get a free perpetual license that can be used on up to three machines. Fill out the form at https://www.querystorm.com/mvp.html and claim your license."
   products: ["QueryStorm Unlimited License"]
+
+- company: "Tower"
+  uri: "https://www.git-tower.com/support/contact"
+  description: "We offer free NFR licenses to Microsoft MVPs for "Tower", our Git desktop client for Windows and Mac. MVPs can simply and informally get in touch with our support to claim their license."
+  products: ["Tower"]