Add a option "file associations",please about notepad2-mod

[zufuliu]

When file type is associated, Windows explorer display file icon use Notepad2-mod's icon, unless Notepad2-mod comes with icons for each file type (which is unlikely), this might not useful.

If you want to always open some type of file using Notepad2-mod, in Windows explorer's content menu chose "Open with" -> "Choose another app" -> "Always use this app to open xxx files".

If you want use Notepad2-mod to open all text files, you can simply add Notepad2-mod to Windows explorer's context menu as https://github.com/zufuliu/notepad2/wiki/Open-with-Notepad2.

[leoxxx]

This function doesn't mod icons in this version which I am using now. So Notepad2-mod neen't come with icons for each file type. It's only easy to open a type of file which your selectd with Notepad2.