Add a way to disable text without using the comment feature

[vkbo]

This relates to #1042.

The comment feature is not intended to be used to take text out of the story without deleting it. However, it is a natural way to use it. There's nothing wrong with this, but when comments are included in the build, they are prefixed with "Comment: " and the excluded text will appear in grey.

It makes sense to have a modifier for the comment code to treat these separately from actual comments. Personally, I use comments to indicate certain aspects of the flow of a scene so I can visually see the balance between action and reaction, for instance. If I take out a paragraph, I don't want that to clutter the view,

A modifier should be as simple as adding a second symbol to the % tag to distinguish it from an actual comment. Ideally, it makes sense to use %~, as ~~ is already used for strikethrough. However, the ~ symbol is a bit clunky to type on many keyboard layouts. I will of course add a keyboard shortcut to toggle it.

In the build output these comments should have their own switch to be exported, and the text could be marked in some way. Possibly with a set of formatting options like red font, strikethrough, or a label.