Allow for 'always-on' plugins

[jamos-tay]

Follow up from 474, 495

Add a mechanism for 'always-run' plugins i.e. without having to include it in plugins, this will apply to plugins such as anchors, syntax shortening that the user should not need to turn off.

Allow disabling by accepting "off" in "pluginsContext", similar to ESLint
https://github.com/MarkBind/markbind/blob/master/.eslintrc.js