Isabelle language - fixed lexer and added sample

Also, Isabelle is very polular in academia.
See for example http://scholar.google.de/scholar?q=isabelle%2FHOL

In around 40 days, the seL4 microkernel [1] with its
Isabelle proofs is (probably) released on github [2].
[1] http://sel4.systems/
[2] https://lists.cam.ac.uk/mailman/htdig/cl-isabelle-users/2014-June/msg00011.html

lib/linguist/languages.yml

@@ -989,6 +989,7 @@ IRC log:
 
 Isabelle:
   type: programming
+  lexer: Text only
   color: "#fdcd00"
   extensions:
   - .thy