Bundle peg-multimarkdown

I no longer seem to be able to install node-multimarkdown due to the fact that the upstream peg-multimarkdown aren't available any longer. For the sake of convenience, I simply bundled the peg-multimarkdown source in the repo to avoid the problem (both projects are GPL-licensed, so this shouldn't be a problem).

Here's a pull request, if it's any help.


—Mt.