add language recognition for JFlex grammars

JFlex is a lexical analyzer generator for Java, see also
http://jflex.de or https://github.com/jflex-de/jflex

lib/linguist/languages.yml

@@ -1434,6 +1434,14 @@ J:
   tm_scope: source.j
   ace_mode: text
 
+JFlex:
+  type: programming
+  color: "#EBCA30"
+  extensions:
+  - .flex
+  - .jflex
+  ace_mode: text
+
 JSON:
   type: data
   tm_scope: source.json