Add development version of telegram desktop

Followed style of telegram cask, but removed (obsolete?) appcast because I wasn't sure about it.

Casks/telegram-dev.rb

@@ -0,0 +1,12 @@
+cask :v1 => 'telegram-dev' do
+  version '0.8.40.dev'
+  sha256 '8ab0de90a581da1685e8ff3224ccf286dd74bb2dde241a69917f1eafd55d8e66'
+
+  # tdesktop.com is the official download host per the vendor homepage
+  url "https://updates.tdesktop.com/tmac/tsetup.#{version}.dmg"
+  name 'Telegram'
+  homepage 'https://telegram.org/'
+  license :gpl
+
+  app 'Telegram.app'
+end