Align how the theme handles cover images

[compleatang]

The internal templates (e.g., _internal/twitter_cards.html) have a divergent methodology for handling the cover images than the theme does.

As currently structured the them is also quite opinionated that images are located in the img dir. Removing the img/ in the cover allows for a more flexible positioning of image files in the static directory, but also (and for my purposes more importantly) allows me to DRY up my yaml front matter.

For me, and I suspect most users, it would be most frequent that the cover image would also be the image we'd want in twitter cards and opengraph (facebook/linkedin) sharing. With the current system my front matter looks like this;

cover: 2020/test.jpg
images: 
- img/2020/test.jpg

However with the minor (admittedly breaking) change below I can do this:

cover: &image img/2020/introduce.jpg
images: 
- *image

I also template those anchors in my archetypes/post.md.

For me the above is much more helpful that it is DRY. I can also see that this would be a breaking change for users so I thought I would propose it with no expectation that it gets upstream.

[rhazdon]

Hi @compleatang, thank you very much for your contribution. I think, this is a very good point and I would also like to merge this pr. I think, this is a very good improvement overall.

I will add a small section into the README to notify the users about this change.

[compleatang]

Great! Thanks!