Added language Isabelle

Isabelle is a generic proof assistant. It is comparables (to some degree) to Coq.

Used in
* diekmann/topoS
* 3of8/sturm
* formare/auctions
* larsrh/hol-falso
* dpthayer/MetaProof

Hello Wolrd example (file must be named HelloWorld.thy):
  theory HelloWorld
  imports Main
  begin
  (*put content here*)
  end

lib/linguist/languages.yml

@@ -987,6 +987,12 @@ IRC log:
   - .irclog
   - .weechatlog
 
+Isabelle:
+  type: programming
+  color: "#fdcd00"
+  extensions:
+  - .thy
+
 Io:
   type: programming
   color: "#a9188d"