Postprocess recursive start at

[skewballfox]

I added a postprocessing step to recursively add Notes linked from the start_at directory to a specific depth, along with a command line flag. I've updated the docs as well as made a test for the new postprocessing step.

Note this could be made more efficient by altering lib.rs to include the ability to set the filter_op like so:

pub fn set_filter_op(&mut self, filter_op: impl Fn(&'a PathBuf) -> bool + 'a) {
        self.filter_op = filter_op;
    }

though I avoided making any changes to lib.rs, so right now it does reparse all files at each iteration step.

If there are any changes I need to make, please let me know