Allow disabling of markdown/formatting preview

While the formatting feature is nice, it actually disrupts me big time as a power user, especially when it comes to server administration. I may wish to see the :emojitext: for example to know exactly which one I'm getting when there may be slight differences between multiple. It can also be somewhat visually annoying to do large message formatting, especially since we don't have a scroll box or a way to fly out the message box to have a larger preview window.

This is more of a personal preference thing than anything and won't suit the majority of users, but for me being able to disable it would be nice.