Add built-in set_user_setting command (#414)

This commit adds built-in command, which is supported by all ST3/4 builds.

Author: deathaxe

plugins/command_completions/builtin_commands_meta_data.yaml

@@ -591,6 +591,13 @@ set_setting:
     value:
   command_type: text
   doc_string: Modifies a view setting.
+set_user_setting:
+  args:
+    file: ""
+    setting: ""
+    value:
+  command_type: text
+  doc_string: Modifies a user setting in .sublime-settings file.
 show_about_window:
   command_type: application
   doc_string: Display the About dialog.