Pull in linked images and assets automatically

**Images and links to local files in markdown are now converted, and assets are properly pulled in.**

Any local files linked with the standard markdown syntax are now pulled in alongside the site, and any references to those images are changed to be the proper URL for the final export location. This includes links to other pages in your documentation. So, if for instance you wanted to link to your "FAQ" page, you can simply do the following

    Check out our [FAQ](faq.md)!

FruitJS will convert that link to point to the final rendered HTML page, leaving it looking like this in HTML.

    <a href="faq.html">FAQ</a>

The same treatment will be given for any images, add automatically add them to the exported site as well. So if you link to an image from your markdown, you no longer need to put that item in your manifest file images list, as it will be added automatically.

## Todo
 - Put them behind a flag
 - Refactor and clean