Fix codefolding preprocessor configuration

[juhasch]

Adresses #1047:

Adds configurable traitlets CodeFoldingPreprocessor.remove_folded_codeand
CodeFoldingPreprocessor.fold_mark.

[jfbercher]

Perhaps adding a preprocess method would prevent calling preprocess_cell for each cell.

def preprocess(self, nb, resources):

        if self.remove_folded_code: 
            return super(CodeFoldingPreprocessor, self).preprocess(nb, resources)
        else:
            #print("Applying CodeFoldingPreprocessor: disabled")
            return nb, resources 

[juhasch]

Thanks, I guess this doesn't hurt.

Maybe I should change the unicode symbol '↔' for folding back to an ascii one '<->'.
The pygment highlighter (rightly I guess) marks it as an arrow.

[jcb91]

is this check necessary, given the check in the preprocess method above?

[juhasch]

True.