135 add function to insert md links

[maw]

This is for #135, should be mostly self-explanatory.

I didn't include a rebuilt zetteldeft.el here. I can add it if you want it though.

[EFLS]

Looks relevant. As I don't use Zetteldeft with Markdown notes myself, what is exactly the benefit of formatting this as a Markdown link? I'm just wondering whether this leaves enough flexibility for users with Markdown notes. I feel hesitant to steer those users in a specific direction.

Here in the zd-tutorial (https://github.com/EFLS/zd-tutorial/blob/master/2020-04-17-1532%20Zetteldeft%20and%20Markdown.org), for example, I suggest a way to use Zetteldeft on The Archive style notes, which look like [[2022-08-05-1212 This]] (i.e., w. a link suffix). Not sure whether this PR would make things a bit more complicated.

Perhaps I can include your suggested code in the zd-tutorial?

[EFLS]

Or maybe we include this, but an 'The Archive' style as well...

[maw]

@EFLS to me, it just feels more "markdowney". Other people might disagree.

During the past year and a half (!) that I've been using zetteldeft I tried out various ways to include or display links and this is what I settled on. (The downside: I occasionally click on these links by mistake.)

I think including it and also adding a 'The Archive' style function as well would be good.

In theory, long term, I'd worry about the list of options for zetteldeft-insert-link-function getting unwieldy -- although it also seems like it'd be a good problem to have: lots of contributions, diverse uses and users, etc. 😄