Discover rel="payment" links.

Prompted by #2, but since this suggestion should probably not be the primary means of determining a payment URL (favouring something explicit in package.json would be better in my opinion) I've moved it to this issue to avoid adding noise to #2.

In the wild there's [a microformat to make payment URIs discoverable within a webpage][1]. The gist is that any link or anchor with a `rel="payment"` contains a payment `href` (and ideally also a `title` attribute to describe it) which resolves to a means to donate. I've made a scraper for these in [this module][2] (only checks HTTPS pages by default).

The package file contains a couple of opportunities to use this. The `homepage` field would be useful for project-level donations, and [author info which includes a URL][3] could be used for personal donations.

Pending the outcome of #2, and if you think it's an idea worth persuing, I'd be happy to open a pull request.

[1]: http://microformats.org/wiki/rel-payment
[2]: https://www.npmjs.com/package/rel-payment
[3]: https://github.com/feross/thanks/issues/2#issuecomment-364351110