add more markdown extensions

[noraj]

Hi,

I'm using .mkd for all my markdown files because .md is historically for Modula-2. I prefer .mkd over other alternative extensions because it is shorter and clearer.
So each time I install vscode on a new machine I'm adding

    "files.associations": {
        "*.mkd": "markdown"
    },

to settings.json.

I think this extension (as some other) should be added and recognized by default: https://superuser.com/questions/249436/file-extension-for-markdown-files

I'm also asking myself if it is relevant to keep .workbook as it is not standard but used only by one app: Xamarin.

I don't know if it is the right file to push that change.

[mjbvz]

Thanks!