Conceal enhancement

[BananenRitter]

Conceal is great but:

1. We should be able to define which parts should be concealed
   For example i like superscript and subscript to be concealed but not things like \left

2. The ability to automatically deactivate conceal when we switch from live preview to source mode

[nexaitch]

Seconding point 1: I really want to use conceal but the fact that some invisible characters are hidden entirely (such as \quad) is really quite a dealbreaker for me

Maybe I'll come up with a fix myself if it starts bothering me enough but something that would fit my usecase is a list of control sequences to ignore in the settings