Update for MultiMarkdown 4

MultiMarkdown 4 has a new github repository.  Additionally, the makefile has been changed slightly.

I updated the formula to point to the new repo, but I also had to change the cflags and make command portion slightly to get it to actually compile on my machine.  These changes may not be optimal for all users, so I welcome any improvements.  

But MMD 3 will no longer be maintained moving forward, so it's important to migrate the homebrew formula as well.


Thanks!