[FEAT] Custom cloze tokens

Currently the plugin provides ==highlights==, **bold text**, and {{curly brackets}} (d8c0e9e) for clozes. It might be better to allow user-inputted characters as the opening and closing tokens like the separator options now.

However, currently you can enable all three cloze options simultaneously, unlike the separator options where you can only specify one separator, so this might need to be handled by either:

1. Multiple cloze tokens can be specified (perhaps extend this to the separator options)
2. Add the custom token option while keeping the current three options (maybe deprecate the latter later)

I am willing to put the work into implementing it but I want to see if this is a good idea first.