MD048: Make it automatically fixable

[skwde]

The description of rules at https://github.com/DavidAnson/vscode-markdownlint#rules is followed by the list of fixable rules.

MD048 seems fairly easy to fix (I also haven't found info on why it is not fixable).

So can it be done or is there a good reason why it is not?

[DavidAnson]

This looks like an oversight. I'll implement this soon.

[DavidAnson]

This isn't implemented because it requires a single fix that touches both the open and close of the fence (different lines) and that is not easily possible today. It will be possible someday and this behavior should be straightforward to implement then.