Added more text extensions and give the ability to add in the config.

Using the 'idf_extra_text_ext' configuration variable you can add more
text extensions.

src/IDF/Views/Source.php

@@ -401,7 +401,8 @@ class IDF_Views_Source
         if (0 === strpos($fileinfo[0], 'text/')) {
             return true;
         }
-        $ext = 'mdtext php js cpp php-dist';
+        $ext = 'mdtext php js cpp php-dist h gitignore sh py pl rb '
+            .Pluf::f('idf_extra_text_ext', '');
         return (in_array($fileinfo[2], explode(' ', $ext)));
     }