Flexible source path for Makefile

[jjolly]

I love this layout. @owickstrom, you're a god! So I expect I'll use it in several situations. But to do that, I want to have separate index.md files.

I've made a change (PR#16) that allows me to change the source directory without having to change the Makefile. The default behavior is to use the demo directory, so nothing has changed for the default, but this makes it easier for me to shift gears.

It helps me, so I thought I'd share. Thanks for all your great work.