Remove IMG support.

Removing left-overs of markdown images.
Cleaner mixin.

Everything uses .indent, whether you want to use padding or margin.
Use Setext

Use underlines for both H1 and H2.
Fixing underline.

Should have spend more time testing.
Allow atx style headers.

Not too happy about the code yet. Use .markdown-atx to use # for all
headers including H1 and H2. Updating the demo with a toggle link.
Removing SASS alternative.

I originally adopted SASS for a cleaner mixin, but I am happy with the
new LESS mixin.