Add support for custom handlers

[sethvincent]

Initial attempt!

This seems like a bit of a hack. Because of the recursive nature of the all and one functions it's a bit tricky to pass the options through.

Very open to suggestions!

[wooorm]

How about we set options.handlers on h in the toMDAST function?

h.handlers = xtend(handlers, options.handlers);

...and use h.handlers in one instead of require('./handlers')?

[sethvincent]

Oh, yeah just like h.augment! Thanks, I'll update this.

[wooorm]

Awesome! 👍