Add meta og tags

[monkeywithacupcake]

changes all templates in Pandoc (html) and skeleton (rmd) to allow for automatic meta/og tags without installing another package.

closes #32

relies on the already included title and image information.

Using this in the yaml:

meta:
  description: "data.coffee.run"
  url: "https://monkeywithacupcake.github.io"
  twitter: "jessachandler"

**results in this in the html header: **

 <meta name="description" content="data.coffee.run" />
      <meta name="twitter:description" content="data.coffee.run" />
      <meta name="og:description" content="data.coffee.run" />
      <meta name="twitter:title" content="Cheers!" />
      <meta name="og:title" content="Cheers!" />
      <meta name="twitter:url" content="https://monkeywithacupcake.github.io" />
      <meta property="og:url" content="https://monkeywithacupcake.github.io" />
      <meta name="twitter:image:src" content="https://monkeywithacupcake.github.io/jess.jpg" />
      <meta property="og:image" content="https://monkeywithacupcake.github.io/jess.jpg" />
      <meta name="twitter:card" content="summary" />
      <meta name="twitter:creator" content="@jessachandler" />
      <meta name="twitter:site" content="@jessachandler" />

and this twitter card:

[hadley]

This would be a good place to try a template partial: https://pandoc.org/MANUAL.html#partials

[gadenbuie]

In addition to og:image, I recommend that postcards also support the og:image:alt and twitter:image:alt tags as well. One option would be to add image_alt or meta.image_alt (or both) to the YAML:

meta:
  description: "floatplane data artist"
  url: "example.com"
  twitter: "yourname"
  image: https://example.com/social.png
  image_alt: Short description of social card image.
image: avatar.png
image_alt: Short description of avatar image.

The two tags to add would be something like

<meta name="twitter:image:alt" content="$if(meta.image_alt)$$meta.image_alt$$else$$image_alt$$endif$"/>
<meta property="og:image:alt" content="$if(meta.image_alt)$$meta.image_alt$$else$$image_alt$$endif$"/>

[gadenbuie]

Relatedly, image_alt could be added here

postcards/inst/pandoc_templates/solana.html

Line 68 in daad882

<img src="$image$" style="height:15rem" class="rounded float-right">