extend no-front-matter to filter

[WeissP]

This is a demo to solve #202.

The idea is to change the keyword "no-front-matter" in dynamic block denote-files to "filter". It acts the same as before if a natural number is passed. To simulate :no-front-matter t in this implementation, one can pass :filter 'no-front-matter. Additionally, one can define a custom function which receives beginning-of-contents to do custom filtering, for example:

(defun denote-only-summary (beginning-of-contents)
    "Only keep contents before the first heading"
    (delete-region
     (1+ (re-search-forward "^$" nil :no-error 1))
     beginning-of-contents)
    (when (re-search-forward "^\\*+ " nil :no-error 1)      
      (delete-region (pos-bol) (point-max))
      )
    (delete-blank-lines)
    )

#+BEGIN: denote-files :regexp "my-note" :filter denote-only-summary 

#+END

Note that currently the documentation for the parameter FILTER is missing as it is just a demo and the feature might change later.

[protesilaos]

I have not had the chance to review this. Sorry! We will do it after the release of Denote version 3.0.0.

[protesilaos]

I never managed to work on this at the time and now think we need to refactor how we approach the dynamic blocks. I will close this but agree the idea is good.

Discussions on what we need to do:

• A general way to exclude files matching regexp from dblocks and maybe more (was: Exclude keywords while using dblocks) #421
• [Feature Request] Dynamic block to insert backlink content #452