Switch to new API for opening editor

[d-akara]

This is a change to the open response in Untitled document.
VSCode 1.11 has a new parameter on the API which allows to set the content on opening.

I think this is better for these reasons:

1. This allows the document to open without a previous undo. So if you hit undo, you will no longer get a blank document
2. Since the document is opened in a state of not dirty, you can close the document without the editor giving you a popup asking if you want to close the document.

@Huachao Let me know what you think

[Huachao]

[Huachao]

@dakaraphi Merged, thanks. Really nice change.