Support `FileList` as input for the `seed` method

The W3C `FileList` object has no `map` method, thus it needs to be converted to an array before using.

Otherwise I got `input.map is not a function` with the following code:

```js
var WebTorrent = require('webtorrent')

var client = new WebTorrent()

// `input` is a file input
input.onchange = function (e) {
  client.seed(e.target.files, function (torrent) {
    console.log('Torrent info hash:', torrent.infoHash)
  })
}
```