More options to turn off markdown and sync the markdown setting across the app and the extension

NB. Mac version now rendering too: 8.10.54 (81054022) on PRODUCTION channel.
Hopefully text selection cometh soon, as stated. ;-)

1P_Dave
Obviously, that's not a solution to the unlying problem though. Extensions should honour the setting in the main app, or otherwise have a setting somewhere in the extension itself to do so. I don't want extra chars in my notes –hence being non-MD– and I'm sure others don't either. It used to render these correctly in extensions a couple of versions ago, but now it's basically a regression that's been annoying users for years now. :-/