Rules override for unsaved files

[OldStarchy]

Feature request, kinda similar to #93, but not really.

I often open up temporary markdown editors to take notes on what I'm currently doing and make todo lists. In those cases, I want to disable rules like first-line-heading and single-trailing-newline, since I don't often have a heading for these "files" and the trailing newline normally gets added on save, which doesn't happen for an unsaved editor.

I could disable markdownlint with the command mentioned in #93 but I only want it to apply to the unsaved editor, and there are only a few rules I'd like to disable.

[PabloLION]

@OldStarchy, maybe you can use my current work-around: I set markdownlint.run to onSave

[DavidAnson]

I like that workaround for something that has only seemed to come up once in two years.