support seeding entire folders (string path to folder)

I changed [`create-torrent`](https://www.npmjs.com/package/create-torrent) to expose its parsing logic as `require('create-torrent').parseInput`, which means we can eliminate the repeated code in `client.seed` and we get folder support for free!

See: https://github.com/feross/create-torrent/commit/e370debdfcd74614d8b921156c0cbd47ef297823