async get (torrentId) {
    if (torrentId instanceof Torrent) {
      if (this.torrents.includes(torrentId)) return torrentId
    } else {
      const torrents = this.torrents
      let parsed
      try { parsed = await parseTorrent(torrentId) } catch (err) {}
      if (!parsed) return null
      if (!parsed.infoHash) throw new Error('Invalid torrent identifier')

      for (const torrent of torrents) {
        if (torrent.infoHash === parsed.infoHash) return torrent
      }
    }
    return null
  }
Returns the torrent with the given `torrentId`. Convenience method. Easier than
searching through the `client.torrents` array. Returns `null` if no matching torrent
found.

@param  {string|Buffer|Object|Torrent} torrentId
@return {Promise<Torrent|null>}