Add button to disable all wysiwyg features

[andynd]

Is your feature request related to a problem? Please describe.
I can no longer copy/paste or write raw markdown without it getting escaped

Describe the solution you'd like
I do not want any wysiwyg functionality, I just want a plain texteditor. Some persons using the same instance do want wysiwyg features. Please give us an option to make both happen and settings to select a default.

Describe alternatives you've considered
Switching to another Plugin, but then I loss the option to switch on the fly or open files with a simple klick.

[pierreozoux]

I personnaly like the experience that codiMD provides. But I understand that it shouldn't be the default for most users.
I personally prefer raw MD too.

this is linked to #418

[edonnachie]

Given that the text addon is regarded as the default interface, the ability to turn off WYSIWYG would appear to be a basic requirement. Or at least prevent the interface from "correcting" quotes, brackets etc., thus breaking many markdown documents.

[juliusknorr]

Let's continue with #439 (which has some more discussion on that)