Add .cake extension for C#

This adds the .cake file extension to the C# language.

Here is a search in the wild: https://github.com/search?q=extension%3Acake+NOT+coffee&type=Code

Cake (C# Make) is a cross platform build automation system with a C# DSL to do things like compiling code, copy files/folders, running unit tests, compress files and build NuGet packages.

You can find out out more about cake here: http://cakebuild.net/

lib/linguist/languages.yml

@@ -392,6 +392,7 @@ C#:
   aliases:
   - csharp
   extensions:
+  - .cake
   - .cs
   - .cshtml
   - .csx