adds latex cls extension

lib/linguist/languages.yml

@@ -388,7 +388,7 @@ Crystal:
   lexer: Ruby
   primary_extension: .cr
   ace_mode: ruby
-  
+
 Cucumber:
   lexer: Gherkin
   primary_extension: .feature
@@ -1345,7 +1345,7 @@ R:
   type: programming
   color: "#198ce7"
   lexer: S
-  aliases: 
+  aliases:
   - R
   primary_extension: .r
   extensions:
@@ -1625,6 +1625,7 @@ TeX:
   extensions:
   - .aux
   - .bib
+  - .cls
   - .dtx
   - .ins
   - .ltx