Allow for custom libp2p generator in configuration

[jacobheun]

Currently libp2p is being created by configuration when ipfs boots. This makes it difficult for users to do anything custom with libp2p, even just adding transports that require the nodes peerInfo on startup, libp2p/js-libp2p#222.

By allowing users to specify a generator for libp2p we provide more flexibility for them to customize it to their needs.

It would be great if users could do something like this:

const ipfs = new IPFS({
    libp2p: (_ipfs) => {
      /* Fancy stuff here with startup info from the ipfs node */

      return new Libp2p({
        peerInfo: _ipfs._peerInfo,
        peerBook: _ipfs._peerInfoBook,
        config: { /* custom config using the above fancy stuff */ }
      })
    }
  })

Then when ipfs goes to start libp2p, https://github.com/ipfs/js-ipfs/blob/v0.30.1/src/core/components/libp2p.js#L11, we can run the generator which would have access to boot time information like the nodes peerInfo.