Alt tags for images in posts

New figure_tag plugin inserts images and captions from the front matter.

Syntax looks like this: 
``` ruby
{% figure left page.image[0] page.image_alt[0] page.image_caption[0] %}
```

Which translates to: 

``` html
<figure class="left">
  <img src="http://images.example.com/solar-farm.jpg" alt="Landscape view of solar farm">
    <figcaption>
      <p>Photos from my trip to <a href="http://example.com">the solar farm</a>.</p>
    </figcaption>
  </figure>
```

This means that only the front matter needs to be maintained to insert images, and no HTML needs to be coded to generate figure/figcaption snippets. It also keeps the markdown file a lot cleaner and more readable since it's only one short line of code.

All of the posts are now updated to reflect this new feature.