add .C and .H as file extensions for C++

"C" and "H" are two file extensions recognized by gcc as indicating C++
source code. The full list may be found here:
http://gcc.gnu.org/onlinedocs/gcc-4.4.1/gcc/Overall-Options.html#index-file-name-suffix-71

lib/linguist/languages.yml

@@ -190,8 +190,10 @@ C++:
   - cpp
   primary_extension: .cpp
   extensions:
+  - .C
   - .c++
   - .cxx
+  - .H
   - .h++
   - .hh
   - .hxx