[feature] New figure shortcode

[bmcminn]

Write:

[[ figure | src: image/path/here.png | alt: describe image contents | caption: contextualized info about the image ]]

Get:

<figure>
<img src="image/path/here.png" alt="describe image contents">
<figcaption>Contextualized info about the image</figcaption>
</figure>

Option(s):

• Let users write markdown in caption field
• Let users pass custom classes to figure element?