local images referenced by markdown should be copied

[clkao]

trafficstars

[sethvincent]

Just added an --assets option so that we can copy a directory of images or other files into the built site.

example:

minidocs docs/ -c contents.json -o site/ --assets images/

You'll then have a site/images directory and can reference the files in the markdown files as /images/whatever.jpg