Provide an option to not link some markers to Ignore highlight group

[fievel]

Some markers are linked to highlight group Ignore:

• notesHiddenMarker
• notesItalicMarker
• notesBoldMarker
• notesCodeStart
• notesCodeEnd

Which in many colorscheme mean that they are displayed with the same fg and bg color (resulting in not being visible at all). I find this is very difficult for editing the file.

As I can also understand someone perhaps like this way of working, I suggest to add an option such as "invisible_markers=1" which will link to Ignore when on and to other group (like NonText) when off.

For the moment, I worked around the behavior by adding a after/syntax/notes.vim which force relinking those groups to NonText.