User Settings - Appending Options Instead of Overriding Them

With ST3 build 3059, settings stored in the user's `Preferences.sublime-settings` file override matching keys in the global file. For most settings this is perfectly fine, but this could be streamlined for items such as `folder_exclude_patterns` and `file_exclude_patterns`.

Since these elements are arrays, why not simply push the user-specified exclude patterns array onto the global one? This way any updates to the source preferences will still be applied for users who use custom patterns as well.