Create a {figure} tag

Creates a <figure> containing
  * an image with alt text
  * a link to a full size version of image
  * a <figcaption> based on content of tag
  * capability to render that content as markdown
  * CSS class hooks for size and orientation

Uses jsdom and jquery to strip out HTML from rendered markdown version
of caption for use as alt text.
Merge pull request #1 from fofr/figure-tags

Create a {% figure %} tag