Automatically pull in images

Pull in linked image assets automatically, and replace them properly with the right URLS.
Tests for automatic image gathering
Fix multipage navigation (issue #17)
Convert links and pull in assets

Automatically converts links to other pages to point to the right .html file. Also pulls in any locally linked assets and includes them in a new "assets" folder in the export.

Need to put this and the images one behind a flag. May be cases when you don't want this behavior.
Adding a few other markdown file format names
Test files for link conversion
Removing some cruft
Merge pull request #20 from ktsashes/resource-gathering

Pull in linked images and assets automatically