Support filtering on the read directory

Note that I've added a `var mdtree = exports` this was only because I wasn't sure about doing `.filter(exports.filter)` - it felt kinda wrong, so I capture the exports in a local name to the module.

Obviously I'm not checking in projects with .DS_Store files, but they're in my local system so here I can author a custom filter to ignore them.

Also bumping version for you to push to npm and learn.jsbin can pick up the change right away :)

Cheers,

– Remy