Add support of spoilers

Thank you for keeping alive such great app!

The markdown markup in some editions supports 'spoilers'.
Their code looks like the blockquote but with exclamation mark: `>! `.
I think it would be great if we have the button for adding the spoilers as we yet have the button for adding blockquotes.

Start by locating the existing blockquote button and the editor action it invokes. Add a corresponding spoiler button that inserts the `>! ` markup, then verify that the editor produces the expected spoiler syntax.

Written by the indexing model from the issue text.