How do I create a custom editor for VS2015?

https://msdn.microsoft.com/en-us/library/bb166460.aspx

Instructions from Link:

To create a VSPackage using the Visual Studio Package template

1. Create a project with the Visual Studio Package template. For more information about using this template, see Creating an Extension with a Menu Command.

2. Select the Custom Editor option and click Next. The Editor Options page is displayed.

3. Type the name of your editor in the Editor Name box. Type the file extension that you want to be associated with your editor in the File
Extension box. Your editor is available for files with this extension.
The file extension is registered for Visual Studio only, not for
Windows. Type the default file name for new documents created with
your editor in the Default File Name box.

4. Click Finish to create your VSPackage in the folder that you specified.